My guess is that he just said that and wasn't really thinking, you know -- how you sometimes say something dumb and it doesn't really mean anything? I wouldn't read too much into it. Just go and have fun and remember that you're just friends. As long as you don't expect anything more than to have fun with a friend, you'll probably have a great time.
I can totally understand what you're saying about the idea of formal being a turn-off at this point. It's fun to get dressed up, but the whole pressure to have a date thing is a bit much, I think.