Nine is not an uncommon age to start being curious and attracted. My guess is that his "friends" told him about the sites, and of course, he was curious and wanted to be cool with his friends. I wouldn't make a big deal about it, let him think that he got away with something. As suggested, I would promptly put a filter on what he has access to (or he'll be visiting really nasty sites before long).

The best thing a father can teach his son at a young age, is how to engage in friendships with girls his age, and how to respect them as people. If you focus on that, you'll have a foundation to build on when he's really interested in sex.