I'd serious talk to him about it, but don't be angry or nag him. It might make the situation worse. Just tell him how much you care about him and don't want anything to happen to him.
Last year in Halls, we suspected one of my flatemates was doing it. He was always poor, and was holed up in either his or his friends room for hours at a time, and was generally strange after about 8pm. My other flatmate was in my flatemates friends room and a load of them were doing lines of the top of a fridge (God knows how he managed to smuggle a full on fridge into his room but he did). My flatemate was never seen doing it, but it was quite obvious he was and it scared the **** out of me!