Boyfriend Issues

So. Recently I got back with my ex who cheated on me and I'm beginning to really regret it. I had gained some weight before he cheated on me because I was very ill and after recovery that's what happened. I wasn't obesse or anything but it definitely showed because I'm barely 5'1. Anyways. The girl he cheated on me with is supposedly a "fitness model" he met at the gym. I used to be insecure about the gym for this very reason. Now that I have confidence and really don't give a flying fuck who he cheated on me with, he seems to be threatened. And withdraws now that I want to get back into fitness. He told me he doesn't want me to go to the gym. And when I say "don't you want me to be happy and regain body confidence" he just says "no no . No." Obviously he is,thinking about what he did behind my back at the gym. But I seriously don't know if I should dump the guy or what. I'm tired of his shit.