Hi.
Users of crystal meth can sometimes display manic and even violent behavior when high, and their behavior can often be erratic, so I certainly understand why you dislike him so much when he's using.
You can be supportive of your boyfriend and try to help him find treatment options--clinics or groups that might help him (or a school counselor if he is still in school), but ultimately, it is up to him to change his behavior.
Putting up with his use of crystal meth, I'm afraid, may actually be sending the signal that it's ultimately okay with you. It doesn't sound like you're having much fun with him though. Breaking up may be the only option.
I understand this is a tough decision. To find some support while you weigh your options, you might want to check out the addiction recovery folder or relationships and sex folder on the message boards. You'll probably discover a bunch of gURLs who can help you through this rough time.
Good Luck.