Since you and your stepson seem to have a good relationship. I would talk to him and tell him how excited I for them and while I would never want to take away from his mother's relationship with the child I would also like to have one (after all a child can never have enough people to love it and I would remind him of that). Then I would ask him if he's thought of what they will call me when speaking to the child since you would like to have a special name. I'm Nana to all my grandchildren...step or birth. My name is Dianna so one of the steps calls me Nana Fadana since she has trouble with putting Nana and Dianna together. makes me smile every time I think of it! Anyway, be open and honest with him about your feelings and it'll all work out.