I suggest you watch his moves. See if he hangs out with 1 girl more than anybody else. If he does, surprisingly go up to him and say "hey" and observe his reaction when he spots you. He should be nervous as he is with the girl. Then try to get a girl (a friend or just a girl you trust) to make a move on him and make her report if he welcomes your friend's advances or not. If that happens, you should dump him immediately!