How do I break up with him?

It’s not how you break it off is when and I say you should NOW. Right now you just described my last relationship I was also around your age when all this happened to me. Believe me no matter how sweet and nice you want to be about it he will be upset, cry, beg and threaten you. I luckily had an older brother who told me to just cut it off or he will go look for him (my ex). So then I did it fast and quick, of course he called and texted nonstop but I told him “if you want to kill yourself then go ahead I don’t care, and if you want to come near me you will find yourself in lots of trouble I told my friends and brother about this case so if anything ever happens to me guess who’s the first person they are going to locate”. He then told me he didn’t care and he was going to kill himself that night, now two years later I received an e-mail from him apologizing and wanting to be friends but of course I ignored it because I’m not getting myself into that again. So yea from experience I say you get out of the relationship before he turns more violent and it’s too late. Hope I helped, and goodluck :)