Hi sasquatch - sorry to hear about your dx. It can be a shock I know, especially as it was not quite what you were expecting.
It is hard when you don't necessarily agee with the dx too. I don't agree my DS has AS; I think he has dyspraxia with social communication difficulties. The thing that makes this hard for me is that is what the clinical psych. who saw DS said but the review team over ruled her despite none of them ever having seen DS and the OT not being there to put her findings to them.
I think though that part of the reason they gave the AS dx is that they think they are doing me a favour and I am wondering whether the same is happening to you. I wonder if they think he would get more help with the HFA dx. It isn't clear cut, the difference between AS and HFA, some say they are the same thing, some say they are different but I suspect anything with autism in the name may get you more support than AS.
I know it is hard to accept though. Does your DS know? My other concern is that since I am not happy with the dx for my son I am not happy to tell him that is what he has got.
I really don't know if you can challenge and change a dx although I suppose you are within your rights to ask for a second opinion.
I would wait until the dust settles a bit and see how you feel in a few weeks and what the implications are of the dx and then speak to somebody again with your concerns.
In the end so long as your DS is getting the right help, I suppose labels are irrelevant but I understand that it doesn't always feel that way.