Educate him as to why he shouldn't eat so much of it. Show pictures of some old messed up teeth, you could show him this video of someone putting a hard-boiled egg in coca-cola (no link but I'm sure a quick google search will do). When I was younger what helped me change my habits was watching "super size me" the movie.

If you want him to change he has to know why he should change and the benefits from it, imo.