As much as it pains me...
I sort of agree with Jesse.
While reading his post, I'm thinking about how you would monitor in Ronald Reagan fashion... "Trust but verify."
Allow open access but state the limits. Spend a little money, probably twice what any monitoring software might cost. Get an external hard drive that backs up not only his data for safe keeping in the event of a crash but also his IE favorites and... his IE history.
One can go in and change the history files on the the computer. But, you can't change the folder contents on an external drive without disturbing the "Modified" date and times. I'm referring to a system that creates a new dataset for each backup period; preferably daily. Now, he may well remove items from history on the computer but he won't be that meticulous every time, all the time. It's so dang easy for a site to show up that is just not right and be missed.
If he never accesses the stuff, great. But, if he does then it's time to talk. There are a lot of similarities between porn, money and a few other things. When you don't have it, you tend to want it the most. When you desire it but you know it's not going to solve the problem, look for alternatives... healthy ones.
It may not work. But, it's an idea. Sometimes, you just have to teach choices among the "broad path" and alternatives down a "narrow path" with focus and healthier outcomes. The whole idea is to have a kid adopt healthy choices early on and go out of their way to find them. Forcing healthy choices may work while they are young but it won't help them set a personal standard that becomes a part of their character.
And, what's "character"? As JC Watts quoted his grandfather... "Character is doing what's right when nobody is looking."
[/Agreement with Jesse]