He called me momma...

Soooooo my soon to be stepson is 19 months old and he called me “momma”. I don’t want to be an entitled fuck and let him call me that, I really don’t want to hurt my fiancé’s ex’s feelings (she already doesn’t like the fact that we’re engaged) and I really don’t know what to do. My fiancé said if he chooses to call me momma then he’s not gonna tell him no but I don’t want the drama. I’d LOVE to be his mom and I treat him like I would treat any precious child, and I’m flattered that he called me momma. I’m just really nervous... HELP