I understand where your coming from here and we don't always get a say in whom we fall for as well. (but wont go into my story).
you wrote that you believe that he has the same feelings for you and seems like too that he is playing along too the important question is he playing or really means all the flirty convo.
My own personal opinion is that you talk to him, explain yourself, but because this is trick ground, because there are many things that could happen; one would be he doesn't feel the same and its friendly banter = then stop the flirting and try to see if you two can be friends or not. Two would be he feels the same way, wants to see where it goes but keeps his girl friend = then you would have to decide if you want to be a mistress? and you have mentioned that you don't want to disrespect him, her and you in anyway so most likely that option would be out. Three, what many girls would like He feels the same wants to leave his girl and start new with you = this is a highly unlikely scenario especially if the girl friend is a long time relationship, not saying that this cannot happen though.
However the worse that could happen is that you tell him and get embarrassed for looking like a fool, but then everyone needs to look like a fool, I should know, been a fool myself and a few time. You also must think what is good for you, there is nothing wrong with being selfish and go to this guy "look I need to tell you, I really like you, your perfect in my eyes and make me so happy, but we need to sort this out, however I will not cross lines, I don't want to ruin what you and your gf have but for my own sake i needed to tell you" .... something along those lines maybe.
But really you can keep it to yourself forever and keep wondering or take a risk.
- If you wish to know more or want me to explain things Email me happy to help
ANSWER THIS QUESTION