First off you should tell him off. He's your boyfriend. Even though he is supposed to be someone who you can be completely honest to and vice-versa, you two should at least be nice to each other and care about each other instead of brutally insulting one another. Obviously, he is not living up to that end. That's why, you need to talk to him. Tell him that you don't appreciate him calling you fat. If he says that he calls you that because it's true in his eyes, then tell him that instead of just insulting you and making you feel bad, he should be supportive of you losing weight or at the very least shut up. If he refuses to do that or continues to call you fat, then you should break up. It's never worth it to be in a relationship that you aren't happy in. Plus then that would also mean that your current boyfriend is a jerk and do you really want that kind of person in your life?