of course he won't stop unless he wants to stop, but chemical dependency is self-reinforcing and he won't want to stop unless something makes him stop. doing a lot of crystal is not only damaging to your mind and body, it fucks up your life in ways you won't recognize until you stop. like the fact that your friends are all dirty crackheads. or the fact that you sound immensely wack whenever you talk.
if you want him to stop, you have to let him see what crystal is doing to him. it's very difficult to do that, especially if he's still doing crystal. if he can stay sober even for a short time, you may be able to get him to realize what it's doing, especially if he understands the way that crystal tries to get you to do more crystal.
however, if you do crystal (even a relatively small amount), it's not going to work. and if his other friends do it, but you don't, expect him to avoid you and go back to the tweakers. drugs do stuff like that.
eventually something will happen that makes him stop. for one ex-tweaker i know, it took losing his job and not being able to buy a ball every couple days. for another, it took owing money to a dealer and moving back to his parents'. for another, it took homelessness - i guess for some it takes getting caught, or death, or something else that serious.