I understand your pain I am sorry you have to go through this. Some years ago my boyfriend of that time also mocked my stutter right when we were having a pretty tense conversation. I just told him "what did you just say?" after that, without anger, without yelling or anything and trying to play it as neutral as possible. I guess he either expected me to not confront him at all and just let him laugh at me OR go all loud about how insulted I felt so that he could go on to tell me how 'dramatic' I am -that's how manipulative he was. But he didn't expect me to be assertive. And that small question was enough to make him feel ashamed and he just said something like "no, I said nothing" (he was also a compulsive lier and 'gaslighter', but that's another story). In any case, in that moment I knew we shouldn't be together. I don't know your relationship, but him mocking your stutter in the middle of an argument does sound like a very low punch, and if this is how he handles arguments now, how is it going to be in the long run, 5 or 10 years from now? Please think about that and do what is best for you.