Help with Autism Diagnosis

I have a son who is 3.5 yrs now, his development was quite good upto 2.5 yrs (ex he would identify objects, body parts, recite some rhymes speak 5-6 word sentences and while we would be telling a story would be able to answer quetion from that story, was always interseted in reading his books, very cheerful, would love to play with other kids- Infact if I had to take m-chat test at 24 month I could not point a single item that he would have failed), but slowly once his sibling was born he bacame silent taking little interest in his books. He started going to playschool arnd 3 yrs but was aloof & crying (with seperation anxiety once I drop him) as reported by caretakers.Another problem was he did not understand the french language spoken in the school. We were told that he may have mild autistic tendencies, one of the first things that we did was to go to an ent he gave some wax and fluid removal medicine which dramatically improved his responses. He started again with his books, reciting the rhyme he now learnt and started playing with kids in playschool. This went on 1 month. Then suddenly his problem started again with cold & fever he started crying all the time, sleeping very less for abt 15 days. During this period all his tests like MRI, EEG etc were done to check for neurological problem which was ok. Ultimately one ent doc operated him for ear fluid & removed adenoid which were enlarged. Now its about 1 month since, he is hearing all the things, repeating what is said and following almost all commands. He says appropiate sentences asking for things if needed or things like shut up when his brother cries. He is quite imaginative and indulges in pretend play. Also he is very amenable to changes in fact I doubt there will be boys his age who have seen so many changes. He also imitates facial expression and voice tones & thats the to n fro exchange I could think of instead of verbal one. He has always been hyperactive doing one thing or other, has been very good at working out mechanism of how a toy works. One peciluar thing I notice that sometime while asking for things he refers to himself as sweet baby, which we call him sometime. Also while in market he does go upto strangers if he sees icecream or cola with them which is very embarrasing. There was also ticks with his lips after his operation & drooling but he hardly does it now. Now he is not going to playschool but once meets any peers he likes to play things like ringa-ringa roses. There is no repetative behavior or stacking of things that I could think of. He does make good eye contact & shakes hand, wave good bye if asked.I think he must have foregotten all the thing he learnt as 2 yr old, my wife is not in favor of pressurising him too much by tring to teach him. He is also very aware of surroundings and reacts in the sense of smiling back or stopping/ crying if my wife shows some anger through her eyes.
Can anyone advice what signs to look for in this case in his behavior which would be important from autism pov. Also any inputs on helping him learn new things or get him interested will be most welcome. With his kid brother he is mostly indifferent occasionaly becoming angry to snatch things which he may have taken from him.

[Sally44]

You can google DSM IV for autism to see what the diagnostic criteria is for an autistic spectrum disorder.  That should give you an idea of the kinds of difficulties a child has to have to get a diagnosis.
You have also mentioned in your post that he had some suspected hearing loss when he was younger.
You have also said that he seems to have lost skills he previously knew, including some language and social interaction skills.
You mention he refers to himself as 'sweet baby' which is what you call him.  Does he use pronouns eg. I, you, me, she, him, her etc and does he get them right.  
Does he repeat things you say to him or use words/phrases from TV or DVDs he has seen?
You say he appears indifferent to his younger brother.  Is he like that with his peer group?
Was there any particular reason you stopped him going to nursery?
Does he throw tantrums or get over emotional about things and you don't have an idea what upset him.
Does he have any sensory issues in any of his senses: vision, hearing, smell, taste, touch, balance and coordination?
My son also seemed to develop typically until around age 2+.  Then I noticed what I now know to be sensory behaviour and from the age of 3+ he began using echolalic speech.  He is now diagnosed as high functioning autistic.
Something that might help you get a better idea of where the problems are coming from and what exactly they are would be to get a private Speech and Language Therapist assessment.  You need to find one who has experience of speech disorders and impairments as well as experience of autistic spectrum disorders.  They need to be independent (ie. not employed by the educational or health authority), and they need experience of educational tribunal work (ie. legal expert witnesses).  The cost of an assessment such as this in the UK would be around £1500.  But you will get a very thorough idea of what is causing his difficulties from a speech and social interaction point of view.  
The reason you need them to be independent is because anyone employed by the education or health authority is going to know the limitations of the provision they have and they will assess and make recommendations to that level.  You need an independent SALT who will tell you what your child needs and what they recommend to meet those needs.  In the UK if you have a private SALT report that recommends provision over and above what the school can provide the education authority has to 'buy in' that extra provision.  Not many parents realise that.  It is a fact of life that every 'authority' runs on a budget and the more needs they find in your child the more they have to fund him - so it is not in their interests to find out what his difficulties are.  
I had a private SALT report done about 4 months ago when my son was age 7.  He was really struggling at school both academically and socially and I needed to get a clearer picture of what was causing him the most difficulties - was it autism - was it a sensory thing - was it a speech and language difficulty.  And although language and social interaction difficulties are part of being on the spectrum, my impression of my son was that from an autistic behaviours point of view that was not the main problem.  And the report did make it very clear that my son is cognitively higher than average intelligence on non-verbal assessments.  My son has problems with receptive speech and comprehension.  So he struggles to understand classroom instructions and to comprehend the meaning behind verbal information.  His expressive speech is age appropriate.  However he uses alot of words and phrases that he has lifted in entire chunks from TV etc and he uses them appropriately.  He has an american accent and a slightly unusual tone of voice.  He has poor working memory and poor auditory memory - so he finds it hard to work things out in his mind and to remember things said to him.   He learns much better when things are reinforced visually - infact he has a photographic visual memory.  He also has Central Auditory Processing Disorder and 'hears' sounds differently - this means he hears the spoken word differently and this has had a huge impact on his reading and writing ability.  For example he hears the word 'volcano' as 'bokano' and 'map' as 'nap' etc.  So this means that his auditory memory store is full of wrong information so he is going to have to learn the alphabet phonic sounds all again from the beginning with signing articulation to show him the right sound he should make for each letter.  
From reading the private report we got, it was very clear what aspect of his difficulties were causing him the main problems at this particular time.  
I would phone around and get some advice.  Your child is 3.5 - ask them about how they would assess him, is this a good age to do such an assessment or would it be better in a years time etc etc.
There are many things in your post which don't point to autism.  For example he is okay with change.  So is my son to a certain extent.  But trying to turn the TV off before the programme has finished is a no-no.  
You say he has imagination.  Are you sure he isn't playing something he has already seen on TV?  My son also has some imagination and can make up games, but he also has times when he wants to just replay something he has already seen and he wants either to do it all by himself or he wants everyone to comply and say and do exactly as he wants it.
But as you also mention social interaction - the professional who should assess that is a Speech and Language Therapist.  So an indepth report should give you alot of the information you are looking for, and an experienced SALT should be able to tell you whether you are looking at a probable diagnosis of an autistic spectrum disorder.
My son also did not stack toys or line them up.  He does tend to play with parts of toys rather than the whole toy - but he plays with the parts in imaginative ways.  But he does find it hard to play with other children because his language skills let him down.  He cannot communicate fast enough or process what they are saying to him for a 'normally paced social interaction' that would typically happen with children his age.  It is as if his first language is not English, eventhough he was born in the UK.
Not sharing or taking turns at 3.5 I don't think is really unusual.  But it is something you need to work on.  My son is great at it now.  At 3+ he wouldn't let anyone near his toys or games because he was afraid he would lose the toy or they would try to change the game - which is exactly what used to happen.
You can use visual pictures and symbols to reinforce learning - again a speech and language therapist is the professional who would show you that.
What are the things that interest him.  It is best to use those to teach new skills rather than try to get them to stop what they are doing to come and have a mini lesson.  For example my son likes to play war games with soldiers.  I can teach him history, language, tactics, problem solving, counting, taking turns and sharing, etc just by playing these types of games and asking him questions whilst we are playing.  He became interested in chess after seeing it on Harry Potter.  He used to just want to the pieces to fight.  Now he has learnt how to move the pieces correctly.  Now I need to teach him to pay attention to the moves I make so that he can make defensive or offensive moves.  
Don't ever assume that a lack of language or social interaction means low IQ.  Your child can be assessed using non-verbal assessments to give you an idea of his cognitive ability.