First of all I am sorry about that. I don't know if you still have strong feelings for him despite that incident, but your significant other should never make fun of you for a disability that you have. The best advice I can give is to talk to him once things have calmed down and tell how much it it affected you when he did that. If he apologizes and said it was In the heat of the moment, then you two should move on after that, but if he doesn't then you may want to consider action like a temporary break up or something. I've never found a girl to accept me for my stutter and because of that I've been alone most of my life. But I know I would rather have a girl who accepts my flaws then to have one that'll make fun of me even if it was seldom. That's just my not so experienced advice