I think if you are not comfortable with her, don't be in a relationship with her. But he seems to enjoy what he has with her and trying to push him out of that, would most likely only worsen yours and his relationship. As for him telling his mother. It is his mother. He's allowed to tell her who he is with. You could express that you are not in this relationship between the 2 of them and only with him [assuming you are or will be]. It could be that polyamory is not your thing and that is ok too. But if you can't support him with who he is, then that certainly wouldn't be a healthy relationship.
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