If at all he has a girlfriend, then you should stop the flirting. He just wants to play with you and enjoy himself while flirting with you. This is also disrespect to his girlfriend, which is an act that should not be encouraged.
Then he will just be using you to tell all of his friends how everyone knows he has a girlfriend so he will keep flirting with you so you won't stop flirting then you'll look like the bad person so honey just do not make a fool out of yourself!!