Oh my god that's horrible like I really don't want to say anything bad about your dad but Jesus that's really unacceptable. To answer your question, I think it depends on how You're approaching this. From what I've read in your post, I'm guessing you don't want to offend your dad (in my culture this is how we deal with it, we just try to be respectful to our parents as much as possible) but you could try talking with him? I'm guessing he knows you stutter if so, tell him that you're finding it inappropriate and not ok doing stuff like that otherwise he will keep doing it like a normal thing when he talks with you.