Step-dad being called grandpa...?

Ashley
A little backstory: My mom and I struggle to get along. We just have such different personalities that it's very hard to have a relationship with her. She's currently on her 3rd marriage. She dated the man for 1 year and they've been married for 2 years. He seems to be a nice guy but he and I have no relationship beyond casual friendship. He's never attempted to get to know me better and I know nothing about him.
Well my mom has picked out her "grandma name", Mimi. She's asked that her new husband be called Pawpaw. Sorry, I just can't... I have a wonderful relationship with my real dad and he will be called Pawpaw. I don't want to be rude to him but what else can my child call him? He hasn't been in my life long and who knows how long my mom will keep him around. I don't want my kid being confused on who her family is.