He is in mainstream and manages really well. It is a tiny village school (15 in his class) and his teacher is just fantastic with him.
The dramatic change was facilitated enormously by SALT - he learnt every single sound in speech one by one. His babbles had rapidly turned into the correct number of syllables but no correct speech sounds except sometimes the middle vowels, so 'da da doo' might mean 'want a spoon'. I realised he was sometimes saying 'dada' when I was nearby, usually while looking at the floor not me, but I wondered if it was meant to be mama. We worked on 'mm' in SALT and I was thereafter called 'mama', I literally sobbed the first time he said it.
We then did every sound in turn, SALT once a week but practicing daily. Because the sounds were written by the pictures we used, he learnt his phonics at the same time, so started reading just shortly after he started talking. He just started this September in reception but has been going to the year 2 class for his reading lessons and to pick books since last Christmas when he was in the school nursery class.
I used to work as a behavioural/play therapist with children with ASD before having kids so used this daily with him and taught all the grandparents etc to use it too. Hopefully the behavioural therapist will be really helpful.
I really recommend regularly filming your DC, I did this and last week I looked back on the videos and it is very moving to rewatch him developing, and helps keep the faith with all the current issues (toileting for us, argh!).