Depending on whether you want to just prank him, or discourage the porn use altogether, either use the above suggestions, or checkout a content filter like squidguard or dansguardian. Even if you aren't trying trying to start a war with him, but just want to help protect your network from pesky trojan horses and spyware, take a look at squidguard and dansguardian. Since you can block URLs by category, you can filter out advertiser's sites, as well as block sites whose only purpose is to distribute malware - something good to have in place anyway. Then, if and when he gets you mad, a quick toggle from allow to deny for the porn category will get you even.
Moral of the story: Don't get mad, get even…
P.S.: You can set either an internal block page with a custom message, or redirect blocked pages to an external URL. So he would only see the lolcats page when he goes to his porn sites, but everything else would work fine. Oh, and if you are tempted to rub it in further by whitelisting your MAC or IP address, so you can say "I don't know what you are complaining about, it works fine here, see?". For pete's sake, set a password on your computer and a short screensaver timeout. Otherwise, you'll go to use your computer one day, and find your history full of porn, your compy full of spyware, and you'll find yourself wanting to take a clorox wipe to your keyboard...
