Since I am at work I will give simple advice.. Talk to him.
Don't accuse him, don't make him feel guilty about it. (as long as it was not illegal porn, if it was, turn the computer off and you need more help then the forum can give (think lawyer))

There is no virus that will download porn. He was the one that downloaded it, but again my advice is to *TALK* discuss, converse about it.
Don't make assumeptions or assume its wrong.
Learn from him why he did it and use common sense (you will be angery and thats the worst time to talk to him about it)


Take care and I hope you follow my advice.. don't talk while angery but do talk and be rational