text: Not enough info to answer this question.  If he is your son, why aren't you asking him?  I'm sure he would explain his rationale to you.\n\nMaybe he feels like he needs to be there to protect her and, since he isn't married, he is free to do that.  If you are divorced from her, he might feel that he needs to protect her since the world isn't always safe for women.\n\nHe probably pays rent and helps her to afford the place.  Maybe she doesn't have enough money to afford to live on her own.  Many women end up in poverty after a divorce.\n\nAs I said, these are only "guesses" since you haven't given any background information so don't take any of my "maybes" personally.  Just saying you need to ask him since we don't know the entire story.