From the appeal point of view, I wouldn't worry.
The admission authority would, of course, want to point out in the appeal paperwork that the offer of adjustments was made.
I don't think an appeal panel would mind your not taking up the offer, provided you explained to them what a difficult decision it was for you, balancing your son's needs. You would need to point out that any change in routine can be problematic for children with AS, and that having to take the 11+ is upsetting enough, without being 'singled out'.
If I were you, I would base your decision entirely on what you think would best suit your son in the actual test. You have been very wise to keep your options open for as long as possible, but if after careful consideration you think the downside of separate testing would outweigh any possible benefit, and if this is also the school's opinion, then you would have a sound argument at any appeal.