I've been there, done that, too so I do feel for you on this one. First of all, this woman is not even his step-mother, just his father's fiancee. Your son is confused, yes, and trying to make the best of everything and children do, in divorce, try to make things look normal to themselves by thinking of both sets (if now there are two "sets") of parents as both being Mom and Dad. However, to help him sort it all out, explain to him, very gently and lovingly, that since he already has a woman he calls "Mom" (you!), perhaps you can help him come up with a better "name" for this woman. Since she is not yet his real step-mother, but possibly might be one day, maybe he can call her something like Mama So and So (insert her name there, don't call her a so and so) or Mamacita or something cutsie, but 'MOM" is reserved for his real mother and you need not tippy toe around this issue. Kids need to know definites and it is better for him in the long run if he differentiates between her and you. Mom is more than a name, but it IS a name -- and it is YOUR name, far as he is concerned.....or should be anyway. So let him know that he is free to love her and has your approval to love her, but the name of "mom" is yours and yours alone. He won't be emotionally scarred by reserving the name of "Mom" for you. And your feelings DO count. Many years ago, my daughter (11 at the time) came home from a visit with her dad and his wife and told me she called the woman "Mom" whenever she was there, and the pain of that has never gone away and has caused me to resent the woman, as well. I would never allow my step-children to call me Mom -- I always explained to them that they had one Mom, but could call me Mama and my name with it or call me by my name alone or some other pet name they could choose, but Mom was reserved for their own Mom, this out of respect to her as the one who gave birth to them, and this particular mom did not raise them daily -- I did! But it taught them to always show respect for other people's feelings and it did not hurt our relationship one bit for them to call me something other than "mom". Also, have a talk with your ex and let him know that you and your son have decided he will call her something else and when you talk to HIM, you need not be as gentle! Kids need to know that we have feelings, too, and if you want an open and honest relationship with him, it's okay to say you love him calling you "mom" and want it to be his special name just for you. It's okay to tell him that. Now, all that being said, my own mother was hateful and bitter to an extreme about my dad and his new wife, and I would never condone anyone doing that to a child. But this is not being bitter, it is not being hateful -- and it's okay to want and insist that your own son reserves the name of "mom" just for you. Yes, it means more to you than it does to him, but that's okay!