Autism, selective mute or maybe both? Please tell me your opinion.

My son is 3 and a half. He has been going to the same playgroup for 18 months and they have recently asked for him to be assessed by the early years senco. They have asked this because he is not sociable, he never really plays with other children and needs to be directed all the time, they also say that he doesnt like change. However my biggest concern is that he never talks when he is there. Maybe the odd one or two words a week to an adult if they are alone but never ever to children. He wont ask for anything, to go to the toilet and wont tell if he is hurt or upset. He can talk absolutely fine at home, never stops lol
If an adult or family come to our house then after a while he will tend to speak a little not as much as most other little boys his age but a little. It seems to only be when he is around other children. He looks so scared and physically flinches if a child gets too close even at the park.

I know that when he is assessed they will be looking for signs of autism. I have wondered about it too as he does show many traits. My daughter was diagnosed at 7 with high functioning autism and my other son at 5 with autistic spectrum disorder but definately fits description of aspergers better. So i do understand about autism but have never experienced anything like this. People thought u was exagerating til they have seen the difference in him there, it is like he completely changes personality from a happy, smiley, confident boy into a nervous wreck! From what i have read this all fits selective mutism and i am wondering if that is what is stopping him from speaking. If it may be that who would diagnose it?

Sorry so long!

[Sally44]

As you are in the UK I think the best place you can contact is the National Autistic Society.  They have helplines and parent to parent lines.  They can advise on getting assessments and which are the best centres near to you to do so.
I personally don't know how they would assess between the two.  I presume he will be seen by a speech therapist and also a clinical and educational psychologist.  They also do assessments at home and in school to see the difference.  You could also provide video footage taken at home without him knowing so that they can see how verbal he is at home.
Obviously social communication depends entirely on the child's ability to communicate.  Can he play with another child at his home, or does even that cause him to stop speaking?
Do you suspect he has any sensory issues?  Some children can get too overstimulated in the classroom environment and just withdraw into themselves.  
My son, who is on the autistic spectrum, we also very different at home to how he was in school.  He too never initiated conversations, or asked for help, or told anyone when he had hurt himself.  
I would have thought that the same professionals that diagnosis autistic spectrum disorder are going to be the same ones who diagnosis selective mutism, however they may be different individuals if it is a specialism.
And it maybe that which ever diagnosis it is, they may use similar types of supports as selective mutism and autism both have anxiety as a background characteristic.
If you feel that you aren't getting the answers you need, you can look into getting a private report done by a SALT and Ed Psych.  Private reports tend to be much more indepth, but again you would need to discuss exactly how they would assess him if he is refusing to speak.  (I am presuming that with selective mutism the child can still interact and understand all aspects of social interaction).  And there is also the possibility, as you say, that he has either, or both.
If you do get private reports make sure that the professionals are independent ie. not employed by health or education, and that they are experienced as expert witnesses at Educational Tribunals.  And it might also be worth holding off for a few years to see how he does in primary school and how he is supported.  If you feel he is coping and is making an typical average improvement (ie. the gap between him and his peers is not widening - both academically and socially) in an academic year then okay.
But I would contact the nas.org and get their advice on how to approach this.

[Sally44]

I would also advise you to find out from both the National Autistic Society and your Parent Partnership (via your LEA), if and when they do any seminars on the Special Educational Needs process.  This is all about School Action, School Action Plus and then the Statementing process.  It is crucial that your children have a Statement as it is a legal document and everything in it has to be done by school.  However most parents have totally unenforceable and illegal Statements because the LEA has not put them together correctly.  They should be very specific and should quantify and specify in terms of hours and staffing arrangements.  For example they should NOT have statements such as "XXX will receive regular input from a SALT".  It should read "XXX will receive a minimum of XXX hours per term of direct one to one speech therapy from a qualified speech therapist (which equates to XXX hours per week), and this programme will be carried out on a XXX basis by a nominated key worker in school who has observed the therapy session in school."  See the difference.  Once is enforceable, the other isn't.  How can you measure if your son is getting 'regular' SALT input.  What is regular, how often and for how long, who is doing the therapy?
The Parent Partnership will be able to tell you where to contact to get the Code of Practice for the Statementing Process, and that is an eye opener reading that, as Statements should contain 'every need' regardless of whether it is funded by the LEA or the NHS.  And the level of provision should be set at the child's need, not the LEAs budget.  And any flexibility built into the Statement should be for the benefit of the child and not the benefit of the school system or staffing levels.