I have a stepson and someday (many years from now I hope!) I know I am going to be thrilled to become a grandma. Will this be your first "grandchild"? When you talk to your husband's son, be sure to express your joy and excitement for this new baby. Draw him into the conversation and the decision of what you'll be called. Perhaps the comment about his stepfather was the stepfather's call. I've certainly known women who say "I am NOT going to be called grandma!", and this man may have that same mindset. I also had an aunt who said her step-grandshildren could not call her grandma, but she softened with time. :-) So, that comment may have nothing to do with you.
Think about what your stepson and his girlfriend call their grandparents. So often, we tend to encourage our children to call our parents a same or similar name as we used for our grandparents. Personally, I expect that I'll be called "Maw-Maw" just as my mother was. However, I'll be equally pleased with Nana, Grandma or just about anything except Granny or Nanny, lol. I have a friend whose children called her mother Grandy. I always thought that was cute.