Tbh, if you like him that much and there is a mutual connection. I’d let him know this is putting you in a bad situation where you are falling for him and he is still in a relationship, but you gotta keep it in your mind if you pursue him right now, and he cheats on his current gf with you. He is going to do the exact same thing to you in the future. I have the same issue to wanting something I can’t have. He’s getting gratification talking to and still being in a relationship.