Unless you were planning to fold anyway, you should announce that you saw one of his cards, before the action gets to him. Then he almost certainly will fold, and everyone else at the table should be shown the card. If he decides to play, he's going to have to do it with everyone else knowing one of his cards.
Just yesterday in a 10/20 LH game, the guy sitting to my right accidentally flipped over his Ace of hearts right as the preflop action was starting. I'm sure several other players saw it as well, and he decided to play the hand, doing the right thing by leaving that card face up until the preflop action was complete.