What course is he on? Is it mostly girls too? If not, then I'd suggest that he gets more involved with the social side of his course (organising pi**-ups, that sort of thing), and if he's at all sporty, joining a club is a good idea. If he's not good enough for the uni sports teams, then your hall should have them too.
I'd suggest if he joins a club or something, he does it by himself. You're great friends, but maybe he needs to do something where you're not involved. If he's chucked in alone at the deep end, he'll have to talk to people!