Im a 20 year old and have AS so iv been through it. My PERSONAL advice is to let him know he can talk to you at any time about it, and maybe if there is a time where you are on a subject that relates even in a small way try talking then.
Forcing the issue will make him NOT come to you when he has a question or a problem.
Also parents probally dont like hearing this but school kids now days tend to know ALOT anyways.
Its a safe bet he knows what everything is and what it does and about safe sex and anything else you can imagine.
What you can and should do is watch who he is hanging out with(if anyone)and if there is a chance that he is sexually active or going to be that you make sure to tell him about protection and reasons why waiting may be better(try not to sound like a prude or sex is evil though).
Finally the last thing I will say on the subject is monitor how he is around girls just in any situation you can. Because kids with AS already have trouble dealing with others alot of times and this can end up very bad for sexual development in the long run.
To this day I struggle being around people and as a result I have no romantic relationships to speak of and probally wont any time soon.
If this is the case therapy may be a great idea. I know if I had someone I felt I could open up to I would have and maybe things wouldnt be so bad now.