Okay, I'll voice the possibly less popular opinion. Your responsibility is to yourself. You don't have to stay with him and you are not responsible for getting him better or for educating him. You need to take care of yourself. If you do decide you want to try and stay, I think it's of utmost importance to get support from others. Can you involve his family? friends? do you have a support system around you to take care of you, if you need it? If he wasn't always like this, something must have triggered this, and maybe you can help him through it. Whatever it is, though, you shouldn't do it alone, and you shouldn't let him take it out on you. This sounds like a situation that requires professional help. I know that posting here was probably already hard enough, so maybe the next step is for you to find counseling on your own, maybe through your school, before you think about talking to him. Figure out your resources and support network, then come up with a plan to confront him. I hope that there is no fear of physical violence, but if there is, let me repeat again: your responsibility is to yourself first. Make sure that you are safe, and take care of yourself, both physically and mentally. If that means you need to leave him, I think that's totally understandable and no one from the outside can judge. And if you choose to stay and try and fix it, again I hope that no one will judge and that you can find the help you need.
