****HOW TO ASK A GUY TO PROM ON IM!!!!!!!!!****?

So I am talking to this guy on IM on fb anonymously at first (I knew who he was, but he didn't know who I was) and when I told him, he continued to talk to me for 2 hours and probably would have kept talking to me if I hadn't said I was going to bed. I want to ask him to Prom but I don't want to be so forward about it so how can I subtly bring it up? Even though he goes to my school, I don't see him. He's not in any of my cles. Suggestions? Thanks so much! Best answer gets 10 pts!