My son is 6 years old and also has speech delay. He now attends a Speech and Language unit. Unlike your son, I was a full time Mum and Granny also lives with us. So he was showered with language, play and good examples of speech - but still made no difference - so don't be too hard on yourself. Early years are incredibly important as children learn so much by example. I would suggest that you don't ever correct speech, but instead praise and model a good sentence structure/words for him. This lets him know that you have understood him and in return he is hearing good grammatical speech. Try and talk to him in uncomplicated sentences - for example only say 'lets get dressed' rather than 'lets go upstairs and look in your wardrobe and choose some clothes for the day'. Often children with speech and language difficulties also have processing (of information) problems. Some of the synthetic phonics programmes are really good, just remember that he might also have auditory memory and poor visual memory, all of which can come with the speech problem. Your son is still very young and the positive change in home environment could make all the difference. What are they saying are the main facets of his problem? My son has a phonological processing difficulty, poor visual and auditory memory. Even though he hears sounds correctly, he has stored them incorrectly in his brain, so when he tries to retrieve them the initial sounds are muddled!
Speech and language delay does NOT have to go hand-in-hand with autism. The unit my son attends is solely for kids with speech/lang no other add-on problems. I think a lot of your sons difficulties will be solved with nuture and time. Interpersonal skills and social skills are all learnt through watching and modelling others, not necessarily autism. Good luck.
No speech delay isn't always autism. But it is something that needs to be considered and screened for if the child is also showing difficulties in the areas associated with an autistic spectrum disorder. Difficulties with speech is only one part of the diagnostic criteria for an ASD. If the child is also not able to play with his peers at the same level as they do, and if they have a need for routines/rigid behavious/tantrums, lack of imagination, sensory issues etc then it would be wise to screen for autism. And some of the things I have mentioned are not even on the diagnostic criteria for an ASD eventhough most, if not all, those on the spectrum do have sensory issues.
I think that motivation can also give you alot of clues. If the child is motivated to attempt to join in and play eventhough they find it difficult because of language diffiuclties, then that shows a desire to interact and play. Most children on the spectrum have a lack of motivation to play with others, or they use others almost like objects to carry out their wishes - there is a lack of interaction and reciprocal behaviour.
But I also remember wondering with my own son, when does a speech disorder - which is bound to have some impact on social interaction - actually cross over into the autistic spectrum. And I think it is just an accumulation of things that are typical of being on the spectrum. Now my son is 8 and he has recently been diagnosed with Semantic Pragmatic Speech Disorder. This used to be considered separate from an ASD, but now it also is considered as being on the spectrum. However, he is still developing and making progress. His conversational skills are coming on. So whatever the difficulty is, your child will develop and learn and progress.
And as you are going to be at home full time, you will soon see whether he is not that interested in interacting with you eg. he prefers the TV, mobile phone, puzzles and legos etc rather than human company.