Most Helpful Girl
To be honest, I could see him being a bit uncomfortable with you calling her that since you aren't technically a part of their family (yet?). It's probably just a bit weird to him, and it might seem a bit forward if he doesn't know that she told you that you could call her that.
I know it might hurt since she definitely sounds like a great grandmother-figure in your life, but you also have to respect their feelings and thoughts.
The family's impressions of you do matter quite a bit, so I'd go along with it for now. How does your boyfriend feel about the whole situation?
If you can't see why he would be upset over a title, why are you upset over it either? What difference does it make to you, calling her by her first name and calling her "grandma"? Especially if you both know that she makes you feel like family, maybe it doesn't need to be said, since it's implied. I don't know, that's just all my opinion though.
Best of luck. Sorry about him:(