Another point is that (imho) there is no such thing as the "gay community", any more than there is a "straight community" (I may be getting into a bigger topic here). There are gay communities and it's certainly not the case that crystal meth is common across all gay people and all places that gay people go. If your son enjoys going out to clubs with his partner where you live, then he'll probably be aware that substance abuse exists and be able to deal with that appropriately. If he's of the few-martinis-then-a-nice-meal-before-bed community, it's unlikely that he'll be exposed to crystal meth or any other form of illegal drugs.
I don't know where you live, but I doubt he'll be in any more risk in SFO than anywhere else - in fact, possibly less so given that the club/gay community is probably more mature there than in many other cities.