I thought you had to have tickets to the prom. He can't just show up. And without proper attire and tickets, they won't let him in.
I think "no" is a perfectly fine answer for whether or not you are going to pay for his tux. Prom is a reward for sticking with school.
And when all else fails when they spring something insane on you, "I don't know, I'll have to think about it" works too.
