Use your chosen name early and often. This may prompt your stepdad to remind you that he wants to be called Grandpa. If so, that's when the two of you will have your talk about why you'd rather it be "INSERT NAME HERE." In the end, neither of you gets naming rights. Once your baby starts talking, he'll probably blurt out an original moniker with real sticking power, like Goopa or Paboo. Of course, you can always help him practice one of those sounds when Goopa's not around...