Laugh and tell him he’s lost his damn mind. No judge is going to give him full custody of a newborn. The only way he would get custody is if you’re doing drugs or something that puts the baby in danger. If that’s not the case I would just ignore him. You don’t even have to let him sign the birth certificate when the baby is born. If he wants to be on the birth certificate he would have to take you to court and do a paternity test out of his pocket.