We have an ABA programme that runs in school. My DS is in reception.
Your DS needs proper social skills programmes run by an adult with appropriate training in how to teach social skills to a child with ASD. - Are you getting outreach support? Or SALT? they should be making recommendations along these lines. He needs 1:1 support at breaks to teach these.
We started with developing social interest in adults - so does he have a TA? They should be spending time 1:1 just doing fun stuff together and getting him to associate fun with being with another person (this is called pairing). For my DS it started with rough and tumble and tickles and now he lives silly rhymes and actions (which are a bit more age appropriate)
Then they moved onto developing interest in peers. quite frankly it was absent, he had no interest. Ditto his brothers. So they use a reward system and every time he gave an item to a peer, spoke to a peer, answered a peer etc he got a token - and then he got free choice of his activity when got 10 tokens - if social stuff is hard you may need to start with just the last one or two tokens and work up. He needed tokens as he was not interested in social interaction at all he was very content on his own.
Massively praise any social interaction.
Should be having small group work eg might be singing group, or playing a game like lotto, jenga, operation, pop up pirate anything with turns which captures his imagination.
Should be working on sharing activities eg DS used to have to be rewarded for allowing another child to play with a train on the same train track he was playing with. They would give him tokens just for sitting and playing alongside. Once his tolerance of other children had built up they worked on getting him to ask for an item or give an item to a peer. So a jigsaw puzzle is perfect - one child has the pieces and the other does puzzle (start with one with few pieces!) and DS had to ask for each piece. Then they swapped and he had to give the pieces. This way he started choosing to play in the same section of nursery as other children where before he would isolate. Again all the time he would be getting tokens for playing nicely and get a reward (free choice of toy) at end.
Once he started school his 1:1 'borrowed' other children and again pairs fun activities so she would get one child to ride one of the big trikes and DS would have to ride on the back and ask for more or to go faster. Being passive he loves anything really adrenaline fuelled so they would be whipping round the playground at full speed laughing their heads off. Now they are working on more formal games like whats the time Mr Wolf etc. The idea to make DS realise doing activities with others can be more fun than on your own.
It has been a long road with lots of very carefully planned steps taught very systematically. With ABA they record every interaction so can see if DS is interacting more over time.
I know we are so lucky to have a fab ABA programme and much of what I am saying may be unrealistic in mainstream, but then if you don't know whats out there you can't fight for it.
We also do playdates at home and this is something you can try. I know how hard it is to ask other parents for their child to come round - I always feel I have to apologise as in 'can your child come round, mine won't speak to them much or play but its really good for his development'! But if you can identify from school the children who are interested in your DS (there will be some, often girls who 'look after' others) ask them round just for say 1/2 hour to start and do something structured. We always ice biscuits, do painting etc and again we might need to use a token board depending on his interest level, again lots of asking for the icing, swapping colours etc with the peer. Pick peers who are good social models. Often you are pushed towards other children who also have delays / problems, but you need to start with the best models.
Other things school can do is pre-teach games. So chase or tig - DS had to be have it broken down and use 2 adults - one being the other child and one coaching (prompting) DS before he tried to play with other children. Find out what games are going on in the playground and see if can be practised with adults / at home.
At school they often take children into the hall and do small group games where DS has to imitate another child or just dance around / sing actions songs. The children love it. They all say DS is their friend, although I know its really his fab 1:1 who makes them all want to join in.
It probably is easier as he is in reception and its all play based anyway, but you need to push for your DS to have similar small groups. As you say his social level of development is at a 2 year old level.
As for the other children in your DS class, they probably gave up as they got no response (DS brothers don't initiate much with DS as he always rejects them). If the other children had an adult showing them how to interact and supporting your DS to interact back, then things would improve.
Sorry thats a massive post. I suspect the problem is not your DS, not the other children or their parents but that none of the staff have a clue how to help your DS. Why would they, its not taught at teacher training. Thats why you need SALT or outreach to be putting programmes in place. He's not going to just learn social skills, its not going to rub off just by being in mainstream, it is going to have to be taught. And my DS is evidence it can be taught. I don't know how things will turn out and if the children will still want to hang out with him in 2-3 years if he is still far behind, but so far its going better than I could have imagined.
Might be worth starting a thread for books for social skills so you can read up yourself.
I would be asking for a statement to get cover for breaks and lunchtimes, small groups etc
Get them to give you a P scale score for his social skills. As he is not making progress in this area then you can start to collect evidence of no progress to help with a statement.