Another one who would be absolutely furious with the school and would send an email asap to say that you need an urgent safeguarding meeting with the head and senco as soon as school reopens, but not given them too many details to start with, so they can't get their story straight over the break.
I would however write down an account of everything your ds told you, maybe email it to yourself today or tomorrow, so it's date stamped and is a contemporaneous account that you can use later on. I would also write a separate document pulling out all the issues and problems, as bullet points or in a table, so that you can go in and go through every little detail, covering each failing that your son had to endure.
I would also get your son to write his own contemporaneous account if he is able to, separate from the nightmare that he recounted to you on his return. If he is writing it down, different details might also appear.
I would also contact the centre they went to and find out exactly what they were told about your son - as much as they can reveal without breaking confidence so might be tricky - but even just to ask them if they were told that your son had asd, and if so, how much they were told about the special requirements he had, the special risk assessments he needed and so on. If they won't tell you, then put it in writing that you will be contacting them once school has returned after easter in order to get them with the school's permission, so not to throw anything away, and to write down what they remember now... Hopefully they will want to cover their backsides too - so will prefer to say that they weren't properly briefed (if that's the case) so they thought they were dealing with a NT child rather than knowingly treating a child with asd badly.
And I'd also ask to see the risk assessments that were done for your son, given that they pestered you to let him go and knew they were going to have to make special arrangements for him as there were things he wouldn't be comfortable doing, thus they knew that he would have to sit out of some activities and that making him do them would scare him... Unless they were planning to make him do the activities all along on the basis that they knew best (!!) then they should have known that from what you have said that he wouldn't be able to do some activities in which case he is young enough that he is going to need supervision - in which case what provision had they made for him.
It's obvious in hindsight - and if they were saying all the right things in advance that then why wouldn't you believe them - but it really doesn't sound like they actually had made provision for him or his particular requirements at all. 
Just out of interest, Is it one of those trips that everybody is expected to go on and so they would have had to have dealt with one student staying behind if he didn't go, making it tricky for them in school if he didn't go? Or is it one where half or two thirds of the children go so still plenty remain in school so him going or not wouldn't make much difference either way?
Your poor son. It's an incredibly cruel and callous thing that those teachers did to him - and I hope that he is able to recover. And that he gets a proper apology from the school, the residential trip staff and the school teachers that too him.