Have you tried turning on the parental controls in the operating system?  Both Leopard and Vista have them.

Dell's are PCs and running Vista (probably).  There is lots of software out there that will let you block porn. The only problem is it ends up blocking a lot of other stuff.  Take a look at http://monitoring-software-rev…..views.com/ to find out what people like. 

The only issue with some of these softwares is they are then not compatible with other software you are running.  It can sometimes be a mess. 

I agree with joey, you may just want to turn on the operating system parental controls.  If you don't want to turn on the web filtering part you can make it so he can't clear the history.  Then you can know what he is seeing.  Well at least on his computer.  Check out the Accounts menu from the control panel.

Good luck!