Take up the highland bagpipes. They can outplay anything.
Maybe you can have a session specifically focused on making him softer. Everytime he plays to loud, hit him with a rolled up newspaper or something.
Okay, seriously, he probably thinks that his way of playing is best. First you have to convince him otherwise. Then comes the difficult task for him to break his old habits. This is probably not the best comparison, but when I first taught myself pennywhistle, I played English style. Now when I try to play Irish style, it's extremely difficult not to slide into English. The point is that your drummer has to be extremely committed to changing and be willing to put a lot of time into it. It will be like completely relearning. If he's not willing (or able), you'll either have to deal with the noise or get a new drummer.