I'm sorry to hear this. Like others said, create a boundary but it doesn't have to be extreme. I'm sure, like so many others w/ toxic parents, that you would love to have a good relationship w/ your dad so, might be unpopular, but try to be the bigger person in this situation. Don't use absolutes "you ALWAYS make fun of my stutter" or words that describe him "you're such a jerk" versus "you say things that aren't words a parent should be saying to their kid. Look up articles on healthy arguing, calling people out, etc. If you can speak to an speech pathologist, counselor, adult you trust (don't have to answer but what does your mom think?). A good boundary might just be that if y'all are having a conversation and he does mock you, call him out right away. No matter what he says, stand your ground. Give him a chance though, if you want to save the relationship. But, if he keeps on doing it, say you'll call him back tomorrow/later, hang up (or leave, go somewhere else if in person), say "I love you" or whatever thing y'all say before you go to show you still care about him even if he doesn't care about your feelings. That being said, I am not a licensed therapist, and I don't know personally know you. None of us do. Think about what you want to do before you do anything big. Listen to yourself and give grace to yourself and your dad. Update us if you think it would help.