I need help. I have no idea what I'm supposed to do with this guy...?

My opinion is that you should not be friends with this person.. Because well as you said he has a girlfriend and yet he does things very sweet to you like flirting. But the girlfriend could find out and get mad at you and you will be involved in a big situation. Since the guy is not that honest can lie and say you were the one flirting and he did not do anything. It will just hurt more, it is better to forget him. The only thing you are doing is going to hurt yourself more. If you stay friends with him than he might say that he is not dating no more and you date him and lastly find out he is still dating while dating you. Just not worth it. There are better guys out there, trust me cause i've walked through that path. But decided to give up on the sweet charades cause I know the truth. :)) its really up to you. You decide what you want.