Suggest you discuss it with him first and then decide. However, if you're not to be called 'Grandma'do you consider it disrespectful to be called by your name? As long as the child is not disrespectful, I think that is the best solution. All of my friend's children called me by my name because I was the one who felt uncomfortable being called 'Auntie' by those who were not really related to me. It worked and made it easier for them to transition when they became teenagers and adults.
(It's better than the Polish word for Grandma - 'Babcia'= old,old woman/lady...).