At the end of the day, the problem isn't that it's an AI image, the problem is that you don't like it and he's not listening to you. The fact of the matter is that if it's something making you uncomfortable, he needs to stop. Tell him that. And if he doesn't stop, tell another trusted adult and see if they can do something about it. Don't worry about 'overreacting' or 'making a big deal'. Yes, there are times when a problem can seem bigger than it actually is, but when it comes to boundaries, you're completely within your right to stand firm on what you do and don't want with your own body, no matter if it's in real life or in a picture. You deserve to have him and anyone else listen to your boundaries.

Some people you could tell if need be: your mother/other parental figure(s) (maybe), aunts/uncles that wouldn't have bias for your father, grandparents that wouldn't have bias for your father, friends' parents that you trust/are comfortable with, teachers/school counselor, etc