OK - I am probably alot older than you - so I'm sort of feeling very old fashioned here -
but I will say this -
If you are with someone - then make a decision if you want to stay with that someone. If you do - then leave this guy alone. If you don't want to be with your current someone - then please end the relationship. But I would not pursue a guy who is currently taken. He is a big boy and he can decide for himself if he is happy in his current relationship. He needs to make the same decision I am suggesting that you do.

Pursuing a guy who has already told you he has a girlfriend ... personally I just wouldn't go there. That's just me -
On the other side he has a girlfriend and you don't know anything about their relationship and I just wouldn't pursue it. Others might feel different.