Poker tournaments are a pretty young game. There are unfortunately no ultimate international rules. I'm pretty sure you can't do that at the WSOP, but even there I doubt that anything would be done unless the other player objects. The ruling would probably be a minor time penalty.

But it's a home game! If nobody but him is left to act, he should be able to do that IMHO. I also think you should be allowed to show one card if another player is left to act. I have never done it, but I like it when people do that