D.D.
My son calls my 2nd husband dad, because well he has been there since my son was 2 yrs old. His dad, my ex-husband has never been there for him, lives across the country. So when my son went to see his "real-Dad" he had a hard time calling him dad, and when we went to pick him up, my son went running to my 2nd husband calling him dad, look at what I got, my ex-husband got a little upset.
Thing is a child wants that connection, if the child is the one asking. Go for it. Even if he wants to call his best friends mom, mom. She is not his MOM. Would that bother you? He is just making a connection with someone. I call my mother and father in law mom and dad, I would feel weird calling them by their names.
Blessings to ya.