@milla: We called them ‘turnabout’. But seeing as I went to an all-girls high school, pretty much every dance we had was a ‘Sadie Hawkins’/‘Turnabout’. Go figure.
As for the question, I assume you an he are fairly good friends. So the next time you’re hanging out, just say something like “You know, Vladimir, I would love to go to that dance, but I don’t really think I would have fun without you. You want to go together?”
I think anyway you say it, some people may think you have higher intentions than “just as friends”, but don’t sweat it. If he asks you something like, “What? As a date?” , then you can clarify.
I once asked my guy friend to go star gazing on Valentine’s Day. It was absolutely as friends (we both had significant others), but he was the only person who would appreciate it as much as I. It was a blast, and there was no missed communication.