Hi Sue,
I would say that, not having been to H20, but knowing what it's about that you shouldn't worry too much. I know it's a meat market, and it's not really my scene (yes, I'm a 42-y.o. clubber myself), but that club is not deep in the ghetto, if you know what I mean, and I don't remember hearing about any shootings there. Not that that couldn't have happened, but I know people that go there that love the place, and they are mature people.
I know it's hard to imagine your baby out there on his own, but if you just try to talk with your son and keep the lines of communication open, things will go a lot better for the both of you then if you try to keep him from doing everything his friends are doing (esp. at age 18). Aside from the CP complication, as long as you know who he's hanging out with and trust his choice of friends, you should be able to trust that they will look out for each other.
Let your son know that it's alright to call you if there's any problem--even if it's 3 AM. It's better to know your child needs help and to be there for him than to have him seeking help from strangers, bec. he's worried about upsetting you. Believe me, I've put my parents through a lot of stuff, and we are still very tight, bec. they know who I am as a person and know that I can handle myself in all kinds of situations.
Of course, I did totally rebel at age 16 and left my control freak mom to go live with my dad (yes, I walked three miles barefoot at 11 PM--of course, it was summer), but I was stubborn, and my mother tried to restrict my every move. Things are better between us now, but if our relationship was different, I would have responded to her better. Hopefully, things are different at your house.
I do recommend the movie "College Road Trip" that's out now, with Martin Lawrence and Raven Simone. It's pretty funny. Maybe you could see it with your son and have a talk afterward.
Good luck with your situation. Hope it all works out.