Maybe try explaining to him just how harmful these jokes can be for you, and if he still doesn’t stop, which hopefully doesn’t happen, then he doesn’t appreciate or care about you as much as he should. Parents should never try to make their children feel bad for something that is completely out of their control, especially for their own entertainment (which is what mocking is). My parents are also divorced, and it can be tough switching between houses, and even more difficult to express this emotion with a stammer. I don’t know if you switch houses regularly or not, but if you do maybe you can ask to stay with your mom more or even full time if your dad doesn’t improve (in my experience they never do). If your dad can’t accept your stammer as a part of you, then he doesn’t deserve your time.