Have you talked to him about the drinking? What does he say? Some people become "situational" heavy drinkers--is he depressed? Does he like staying home or is he resentful about it?
Even though his brother might have given him a "drinking buddy," it might be that he's lonely and drinking with his brother gives him some adult contact (dysfunctional though it may be).
Or he COULD be an alcoholic, or on his way to becoming one. It's really hard to say.
I'd have a chat with him about the drinking, though. If all you do is kick BIL out, that's not likely to solve the problem.