This all depends on your level of computer knowledge vs that of your son. If you don't know what you're doing and he is more computer savvy than you, then chances are he'll find whatever you've put in place on his computer and disable it or bypass it.
If you want to go down this path, my suggestion would be to set up a machine as a proxy server (using something like squid) and route all your web traffic through that. Then you could have log files which would show which sites he had connected to and when. Because it would be running on a separate machine, he would not be able to access it (you would need to secure the machine, obviously). That would mean that he would not be able to disable or bypass it (assuming that everything else, like your router admin screens, are also locked down).
The alternative (and the advice I have given to my friends) is to keep any network-connected computer in a family area (such as the lounge or living room). That way you can monitor it simply by looking at the screen.
However, I don't think either of the above options are a good thing to do. The chances are that any technical restriction you put on your son will just result in your son looking for a way round that restriction. There are many ways to get online which require no technical expertise, for example going round a friend's house, going to an internet cafe or using his mobile phone to surf the internet.