Try changing the history settings so that it records a few days' worth. If he doesn't check the history settings, you may catch him off guard and get a log of his activities! If he keeps resetting it to clear the history, tell him not to. If he keeps doing it, maybe he is hiding something?
You could also try going into your Windows folder and looking in the Temporary Internet Files folder (if you're on XP etc it may be in a different location). This contains files recently viewed by the computer. Most of it will be junk eg button pictures which aren't much help but some of it may be porn, if viewed recently and not cleared out. I can't give you much better advice because I have never tried to track activies like this before.
Net Nanny may help to block porn websites and control his activities but I would try talking to him first. I'm no relationship counceller and I'm not telling you how to father your son, but I think a good talk about the subject with him first might help.
