I can see why at 18 he doesn't really want people talking to him on their terms, if you know what I am saying. Teens usually do not like being shoved into these kind of conversations. The rule of thumb people use on here for those with milder AS is that they generally are emotionally about 2/3 their chronological age, which means you can expect a young teen type approach of not wanting to talk to adult about sensitive things..
Also if he did not do well in community college, he probably feels like a failure and these kinds of conversations would make him feel defective.
I think the best approach, especially for someone he sees occasionally (assuming you are not asking on behalf of his parents) is to talk to him about special interests, and things that give him positive self-esteem. He may be depressed and so forth; but I think as an uncle the best thing you can do, is be there for him in a positive way, without bringing up the AS. His parents no doubt, have specific issues. I am assuming he is staying at home and such. If there are specific issues you want advice about, post them; but realize that he has to want help.