What should my son call my dads fiancé?

So my parents got divorced 3 years ago. There had been a lot of space between them for a while and then my dad cheated. I didn’t talk to him for a while because I was upset at him for doing that to my mother, but he’s my dad and ultimately I wanted him in my life. He is still with the woman he cheated on my mom with and they are now engaged. Recently, my mom made the request that she doesn’t want my son to ever call her grandma. He’s only 9 months so not calling anyone anything, but I’m wondering about how others handle this for the future. I only call her by her name and don’t plan on changing that, but my son only knows his grandpa as with her and I feel he may question why he’s grandpa but she isn’t grandma. I don’t want to hurt my mom at all either, she has been great about wanting us to still have a relationship with our dad despite her own feelings about what he did. Do you use another title? Let your kid decide what they call them? Thanks for any advice!