I definitely agree that your son needs to be the one to handle the situation at least initially. But you as parents can help by talking through with him who he should talk to and what he might say.
I'm 50/50 about whether it might work to just distance himself from these guys for now while he waits to see if they either self-destruct or wise up. My initial thought was that he needed to protect himself from "guilt by association" by having a meeting with a coach about this. But I understand if he is reluctant to "rat out" his roommates.
The suggestion to go to an older team captain (or two) with the problem initially makes sense to me. But I wouldn't rule out your son needing to go to a coach to say "I'm committed to following the rules and keeping my grades up, and it's really tough to do that with the living situation I have been assigned to" and then let the coaches ask the questions from that point.
Good luck, and let us know how it turns out for him!
Julie