Should I talk to him about what is going on?

Personally, I think it's cool to play the field and test relationships because you want to know what's out there before you make any serious commitments. I'm not saying that the feelings you guys are feeling aren't real, they are, but in my experience, they're somewhat fleeting... they come and go from person to person depending on who can offer what, so to speak (and that goes for girls and guys obviously if he has a girlfriend and finds you more appealing...) There is no question that this guys also likes you. I would suggest hanging out and befriending him more to see what develops, as you have mentioned. Who cares that he has a girlfriend? If you guys are just hanging out (and possibly innocently flirting), you're not really doing any harm, it's not like you're married :) Only if you notice that things get hotter between you two and you feel uncomfortable about it should you have the guts to stand up for your own morals and DO let him know that you aren't interested in being the girl on the side. You sound pretty smart, so I'm sure you'll follow your instincts. Good luck! :)