Hey there. I noticed that no one has replied yet, so I figure I would.
I'd like to point out that what I'm going to say is not sugar coated.
To be completely honest, I think he is being loyal to the girl he is with. He seems to be an honest man, and it's guys like him you don't see everyday. It's guys like him who are always given a bad rep by the ass holes who cheat and play girls like a board game. I think you should leave him be. Don't try taking him from the girl he is with. It will be hard, trust me, I know how you feel (but with a girl, instead of a guy haha). The only thing you can really do is hope that the girl turns out to be a flop and he moves on from her. Because if that happens, him moving on from her could make him run into you again
Until then, keep searching for a guy like him. You'll find one. God knows I found someone like the girl I liked.