either ask matt or wait and see how they refer to you. Since you have a good relationship he will probably refer to you in an affectionate way.
IMO I don't understand people getting their panties in a twist about a child calling someone"grandma".(I understand resentments/anger due to divorice/abuse play into it) I'm refering just to general circumstances. as long as they are loving, can a child have too many?? My kids refered to their GPs as grandma A or grandpa B when they were talking about them but when they were with them it was just Grandma. My DIL comes from a large blended family and refers to all (8 or 9) grandparents as grandma/grandpa.