Feel son is being left out at school

hi everyone, 

i have a 4.5 year old son who is autistic (although no formal diagnosis as yet.. still waiting!) He started a mainstream school in September and they tell me he is settling in and learning the routine, but they only want him to go until 12, because they feel he won't cope with a full day and the afternoons are more structured. 

Firstly, I asked he could stay later than 12, just so he gets more time for interaction and to eat his lunch (they go in at 11.45 for lunch, so he only gets 15 mins and wonder why he hasn't eaten much!!). The senco agreed he could stay til 12.15, after 1 day I was told he had to finish at 12, because he got a bit overexcited (it turned out a game had started between a few children, so naturally my son got excited with it all).

what also upsets me is when I pick him, he is always sitting at a table alone, I feel like they're saying 'you're "different" to the others, so you should by yourself' 

what my concern is that if they say that the afternoons are more structured, doesn't that mean that's when they do their learning (letters and sounds etc)? Which means he basically goes for 3 hours to play (which I know is still important), but he's not really being taught the same as the others. 

Sorry for the long post and rant!! 

Thank you