I read this earlier but didn't reply. I keep thinking about it and how I would handle it (I'm not a mom...never have been and I didn't go to prom).
You say now it's your way or the highway, I understand that now knowing that you told him from the beginning it would be that way.
But honestly it's his night and should be able to wear what he wants to if your paying or not (obviously he's not MAKING you pay for it). I guess I don't see really see why you guys are so dead set against what he wanted to wear.

And not your not evil, just a little sneaky