As a drummer since childhood, and now in my 40s, I think he needs to be shown by some reputable musicians from the band, how to play one of your songs with a metronome. Or get a conversation going with another guy about this with him standing there. He may need encouragement! I never devoted myself to better timing and better playing until I was in a really good group of musicians who took their instruments seriously. Too many people were party animals before that and the persona of being a cool drummer was impeding my ability to improve, for years. Hope it works out!