If you know his number, call him up one day and chat for a while, then bring the conversation around to the fact that you're interested in him. Try to find out how for certain how he feels about you. If you do end up in a relationship with him, be careful, since if it ever ends, it might make it awkward for the group anyways.
Here's
a good question with a really good answer that hopefully will help you even more with this situation.
Good luck with him, and fill me in on how it goes.