One question I want to ask is, what does your son want?
A lot of adults with ASD really, honestly, prefer to be alone.
My son, age 16, has always been drawn to people, so it worried me as he cut his social circle down. And down. And down. But I can't ignore the fact that he is perfectly content with his life, and doesn't want more close relationships than he currently has. I've also observed enough (recently chaperoned a class trip) to realize that he is capable of socializing appropriately, when he puts his mind to it, and his classmates seem quite comfortable with him. So, I back off.
But if your son does not know how to socialize, or wants a larger circle than he has, that of course should get worked on. Easy place to start is a club or group for a shared interest; many Aspies joining gaming groups. If you have a specialty gaming store in your area, check their schedule.
If there is a lot he needs to work on in the "know how department," I would look into speech therapy. My son got social skills and non-verbal communication training through speech.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).