It can be extremely difficult, if not impossible, to quit using if your social circle revolves around using. Especially with marijuana which has such a strong culture around it. It may be hard for your son to give up his friends and his drug but he may have to for awhile, if not for good, to be able to successfully quit.
As far as AA, there is also NA which he could try as it focuses on using other drugs besides alcohol. You're in a big city so he should also be able to find NA groups specifically for young folks (I remember when 30 was old, lol, now I think it's still being a kid!). His former college probably has some oncampus groups he could also check out. Or maybe another group or program might work better for him like SMART, etc. There are lots of them listed on this website.
Does he do any specific counseling around the addiction? I know you said in your other thread that he does see someone, but going to an outpatient treatment center might be a good choice. He would certainly meet more people his age and get tools for living in recovery.
Or maybe he could come here to SR? This place has been a real lifesaver to me and so many people. Would he consider just reading some of the threads?
In any case, I again just want to say how awesome it is that you care so much for him and want to help him.