Just casually strike up a conversation with him. Eventually move onto the topic of the dance and say, "Oh have you got a partner yet? I still haven't." But remember to say it in a very casual way. Being friends you can do this. He replies that no, he doesn't. "Mmm. Maybe we should just go together then?" Make it sound like a spontaneous decision. Then he says yes blah blah you go to the dance fall in love and live happily ever after.