It sounds like he's depressed and needs some professional help. Please take comfort that there is nothing you can do to help him and by staying with him does not help him either. If he's serious about the suicide thoughts then he needs help, but that is out of your league. No offense meant there to you either, but he needs very good therapy at the very least and you're not trained in that huh?

It could also be manipulation. I mean that he doesn't mean the suicide comments, but is manipulating you into staying with him. This is wrong, and you should be free to live your life your way, and if that means leaving him, then do it. Even if (and that's a huge if) he did ever harm himself, then that is his choice, not yours. You are not responsible for other people's actions. You do not put the weapon or whatever in their hands and please don't feel guilt by being true to yourself.

I would tho, if you can, tell his parents, your parents, a teacher or someone about his threats. Please let them take over the care he needs. You will most probably be doing him the biggest favour you could by doing this, and that is a sign you care about him. You also need to do whatever makes you happy, and concentrate on yourself and your grades.

Hugs to you and good luck.