I can't tell you about senior school help for an ASD, as my own son has dyslexia and it's been a battle to get the right help for him (finally involving a change of schools). But I can tell you what you should be looking for in the school, based on my own experiences as someone with an ASD.
We have to know what will happen at each point of each day. It's not a desire, it's not a whim, it's an absolute must. The school has to be prepared to understand that and work with it. They have to help him structure his day and help him work out what he'll be coping with if there are changes to the plans or the teachers. Each new teacher has to know that he needs to have certainty.
Environment: Do they have fluorescent lighting in the rooms? Might be a real stress-factor for him and cutting across his thinking. Also any noisy social group-work in the classes, or background humming from machinery. It's as loud and distracting as sitting on an airport runway for us.
In terms of social things, if he is on the autistic spectrum, I'd say the most important thing is that they respect his need to be 'him', not expect that if they force him to socialise, it'll train him out of having an ASD. We don't 'do' eye contact or social chat. Well, we learn to do it because it's the only way to make more friends, but it's a massive challenge, and as exhausting as someone trying to climb a mountain. I almost lived in the library at school during breaks and dinner time, and any time I wasn't in the libary, I was on the tennis courts. Team sports were a total nightmare. Tennis a welcome relief, since it was a repetitive fairly solitary sport with no need to do eye contact or master the finer points of what the other person needs you to say to them at each point in time. I wasn't a lot of good at tennis, but that didn't matter to me. It was the repetition and the time to myself that counted. Find a hobby he loves, get him to do it at every possible point in the school day, encourage the school to support this too.
When he comes home from school, he'll probably need some 'down-time'. He will need to say nothing, do nothing. After-school activities will probably be too much if they are straight after school, unless it's a fairly low-socialising hobby.Most important, listen to your child and really take notice of changes in his behaviour. We're rubbish at saying when we're hurting or scared or depressed. I work better with pictures or writing things down than saying words when I'm very stressed, even as an adult. If he finds it difficult to articulate how he feels, get him to design a door poster for his room that has the key words on it so he can just put the right one up there. "Bad day" "Leave me alone" "I hate them". Anything at all that helps say what he temporarily can't.
A buddy scheme works really well in a school. A sensible reliable friend to keep a watchful eye for trouble, a well-policed playground so that bullying can be spotted, and a good-behaviour-reward scheme for the whole school to promote caring and citizenship. Get the school to contact the local charities for support, advice, training etc.
Keep in good contact with the tutor, the SENCO, and anyone else involved with his life. Keep calm. Keep notes. Be firm but fair with them.
Be proud of what he does well. Make sure the school is too.