same case here. I asked C6 once, and he said it is difficult for him to say "Mom" to my mother as he thinks she is not his real mother. (I can understand as I call his mother as 99, I can't call her "mom" as well)
I asked C6 to call my mother "grandma" just to follow my nephew even I have no kids. It is better than he doesn't call my mom at all. I told C6 that it is so not respect to my mother.