Most Helpful Guy
Make sure that his family and friends are keeping a close eye on him. Break up with him but tell him that he can still talk to you when he feels depressed.
The best thing to do is to make distance from him slowly and make sure the people around him are monitoring him.
Most Helpful Girl
Listen to me when I say, you are NOT responsible for any actions he takes after you break up. You should never stay with someone you don't want to be with just because you fear they may harm themselves. You need to do what's best for you and it's not fair to yourself (or to him) to stay together when you don't really want to.
You just have to tell him that you don't think it's working out and that you think you should break up. There's really no other more 'delicate' way of saying it. If he tells you that he will hurt himself, then at that point you need to tell an adult. You can tell either his parents, the guidance counselor at school, etc. so he can get the professional help he needs.