I had this exact scenario a year ago, and like you, I hoped my ASD DS (age 8 at the time) would say he didn't want to do it. And to start with, he did say no - but we were given a year's notice and I wasn't prepared to rule him out so long in advance in case he changed his mind. So I paid the first two deposits to keep his place, accepted they were non-refundable, treated them as insurance. Sure enough as the final date for paying the balance approached, school had clearly been talking a lot about the forthcoming trip (it was a two-night residential at an outdoor centre), he knew his special friends were going, and he said he wanted to. Fine, and I paid, also acccepting that he could always change his mind back again, particularly the fact he wouldn't have his iPad with him really hit home...
The centre was also about an hour away, which was a factor; I was absolutely prepared to go and get him any time. But I was worried about everything: how would he be in a dormitory? would he clean his teeth and wash? how would he deal with dirty clothes? what if he didn't like the food?
It was all fine. DS had a great time, and for the most part, his teachers gave him the support he needed. There were a few incidents that came to light when he got home (his trainers were utterly caked in mud while his wellies were in the suitcase clearly unworn, and it transpired that he hadn't noticed them and no teacher had thought to come and help him find them - as though a parent would send a kid to an outdoor centre in March without wellies), but nothing major.
However, I do think I projected my own FOMO onto DS. At that age, I would have hated that sort of visit but no way would I have said I didn't want to go. DS doesn't really have FOMO.
The children who didn't go on the visit, by the way, had to go to school as normal where they did some similar (indoor) activities as those on the trip; two classes at a time went (of a year group of 4 classes) so I believe the remainers joined with the classes still in school.
I didn't do any preparation for the visit, aside from showing DS on the centre's website, where there were 360 degree pictures/a virtual tour. DS wouldn't have kept quiet about his feelings had he really decided it wasn't for him. And I'm glad he went, even if, to this day, he's barely talked about it. I imagine he stuck like glue to his best friend and copied the other kids, and got through that way, even though I am fairly sure he won't have had a clue what was going, probably didn't change his underwear or clean his teeth...
If you think it would help your DS, could you arrange a visit to the place they're going? Definitely go through all your concerns with school. How long have you got to prepare?