I think scholarship may be something of a red herring, as these days these tend to be relative baubles, financially. Bursaries are what you need to look for.
We are in a similar situation in terms of wanting a 'top school' for a mild AS boy for our son and there aren't that many choices.
Boarding is I'm afraid a very big risk, our EP said for instance that she was dealing with an SEN boy at 1st year at Eton and he was finding it very difficult in terms of the atmosphere. Very many public schools seem to be set up for macho rugger types IME. Many of them not all that bright I would have to say!
Otherwise is Norwich School an option, as a day school?
Oakham doesn't look terribly impressive for the brighter boy, Oundle better.
Otherwise perhaps a train into Cambridge for Perse Upper School as a day option?
I'm assuming other children safely esconced in same prep school on bursaries, so a reluctance to move for that reason... Far more choice if you can move towards London.
If the school has said your ds is scholarship material I would expect them to give clear guidance and recommendations for suitable senior schools. If I were you I would be making an appointment to see the head to discuss your options.
What year is your ds in? Quite a few of the main public schools have pre-tests in year 6 so if your ds is in year 5 I would be arranging to visit those schools sooner rather than later. I know that Winchester close their house lists three years ahead of admission, ie if you were looking for a place in 2016 you would have needed to select your house by summer 2013.
It might also be worth posting this on the SN board. I know there are posters on there who have grappled with exactly that question of what to look for (and what an independent school actually delivers) when it comes to AS.
Might get some general pointers or even a response from someone local to you.