Personal boundaries take time to take place once in a new relationship.. meaning that you've only started seeing eachother in person a month ago. I'd be uncomfortable with it too tbh. Emotional cheating is a thing too and he seems to be doing it. Yeah the girl is 15 but he's said some things to you about her that would make me feel extremely uncomfortable and hurt. I'd wonder what he says about you to her, if they're better than what he says about her. But that's besides the point.. it's pretty insensitive to hold someone to such a high degree and put that much effort in, to get that close with when you're in a relationship with someone else. There should be personal boundaries on his part.. I'd feel uncomfortable with most of the things he does and says if I were you.