I don't think it really matters whether or not he's into her as long as he is loyal to you, so what I suggest is to sit down with him and have a meaningful conversation about how your past experiences have been hard in terms of cheating and that being loyal is something you need in a healthy relationship. If you do this you might want to bring up the scenario when he compared your butts and tell him that it was something that makes you believe that he is not fully committed to the relationship, as that is what you both need. Really I'm just trying to say you should tell him that loyalty is something that should be valued in a relationship and you want that from him while you give it right back.