Without entering in to the whole debate about the club as the parent of an individual with Down Syndrome may I point out the use of "Aspie" is offensive. What would you call my son "Downie?'It could be very useful. Do we know why he wants to join a club? A few years ago when I got heavily back into road biking I thought I might join a club. It kind of seemed like the done thing. I went on a few rides before I joined and I realised that it involved social interaction, strange rules and conventions, talking to strangers, basically all the things I use cycling to avoid. I get out on my bike to get away from people and give my brain some much needed space and freedom from neurotypical conformity. I don't want to hang around with a bunch of strangers that I know I'll never quite fit in with.
So Aspie dude might need to think about why he feels he wants to join in with the club to begin with. It might just not be for him. If he decides it is, fair enough but if he's doing it because of some misguided interpretation of an unspoken convention that that's what cyclists do these days, then he has to think about whether he really wants to and if he might not be happier out on his own.
This is of course quite separate from the issue of their behaviour towards him and the fact that they seem like a bunch of arses. They obviously have some sort of deficit in social and communications skills if that's the way they act.