HEY! don't diss porn... some of its pretty good. as a kid (13 years old) I can say that us kids look at porn, because to us it is the best thing in the world (at our age, if we see it in real life, then, well...) so, if ur kid has seen it, let him see it, brag about it, and itl just die out, rather than restrict him. if you restrict him, he will just try harder to 'thwart' you and look and it, and ull over-emphazize the entire thing. know what im saying?
the best thing... (its working on me right now, im embarrassed as hell about it, oh well) catch him in the act, when hes "doing something" with the porn (masturbating) and threaten to tell everybody. in time, tell as many people as possible, and rub it in. (it works.)