I'm going to tell him that everyone BUT him is having a good time, and that, despite his feelings, he is NOT the center of the universe.
I'm sure we've brought along some personal electronics (which is probably what he'd be doing at home anyway). He can play with his PSP or equivalent, and let everyone else enjoy him or herself. If he can do that, I'll make it up to him later. If that isn't satisfactory to him, I'm sure we can find a more forceful was of convincing him that's his best course at this point.