Our Cubs is brilliant for DS who has AS. There are at least three of them with AS and the leaders are great, very supportive. My tip would be to follow your DS's lead with staying, I fully expected to have to stay with DS when he started as a Beaver but he got stuck straight in and within 5 mins was telling me to go home. I would offer to help out at the meetings, that way you are getting involved and finding out more what happens, also the leaders will appreciate it and you won't just be sitting watching your DS.
I would ask the leaders what experience they have and how confident they are to manage your Ds's needs. The group that Dd3 goes to has leaders with a wealth of experience in scouting and in their jobs.
They are brilliant with Dd3, she has even been away camping with them!!! We made them a communication passport the leaders loved it and said it gave them a clear understanding of her needs.
As Maria said, groups vary a lot. Most will do their very best, but ultimately you know your ds best..... would it help for you to go along to a meeting without him first, so you can be aware of what goes on, what might make him anxious and thereby helping with the transition? Does he use things like visual timetables at school and home ? Would that help at Scouts ? Please do talk to the Leaders to let them know about your boy - being willing doesn't necessarily make them experienced. Do let them know that they can ask you about things they are not sure of at any time. The answer to the question depends so much on how the AS presents in your ds tbh - it's such a wide spectrum.
Yes, I was assuming you have already spoken to them about DS's special needs, if not definitely do that, they are a very inclusive organisation but obviously the leaders vary a lot in their levels of experience.