I can't really judge your situation because I don't know your son, but my prom is coming up in a couple of weeks, and I know that if my mom switched my dress on me, I would FREAK out. Then again, I think a dress is more important than a tux. I think that doing it behind his back wasn't the best way to do it -- I think that saying "This tux, or we're not paying for it" would be better. After all, when he finds out it was switched, it will be at the last minute and he'll have no time to take it all in. If I had found out my dress was different the very day before my grad, I would probably refuse to go just out of spite. So I don't think that having him wear another tux is wrong -- it's the way you went about it. I would have a talk with him.