As a recovering alcoholic myself (15 years in AA), he will hit rock bottom before he figures out that he needs help, and even then, he will only seek help only to silence those who make a fuss about his drinking. While alcoholism may be classed as a disease in some outlooks, it is a selfishness disorder first. Soon, his drinking will drive away everyone he knows and cares for, and he will not blame it on himself. There may be that pivoting moment in his life where suddenly he realizes what's wrong, and takes honest steps to correct it. It's almost a lose/lose situation, where if you don't tell him, he continues his fall, and if you do tell him, you may lose a drummer and a friend if he takes it to heart. I recently left a band and formed a new one because 2 of the members were severe substance abusers. One of them was a really good friend who kept on being pulled down by the other one, even though he was making an effort to improve his life. It was hard. I have a feeling he will lose a lot more before he decides that it is not worth it.
All I can say is that if you are good friends with him and you feel he can endure the rough times that may lay ahead for the both of you, then tell him and hope for the best. If not, then either find another drummer, or make your own band, because his drinking will get worse, and it will harm the reputation of the band and kill the fun.