We've used netnanny in the past. Smart kids know how to get around it...and unfortunately, it sometimes keeps you from getting to good sites...which may be frustrating for your Mom.

A better alternative may be to treat him like an adult (like xrayted, I'm guessing that he's around 18). You can't keep him from doing it, but you can make a pact with him not to do it on Mom's computer...then verify it with site logging software.

Just to be safe, make sure you have good virus protection as well since I'm guessing that many porn sites are really just a conduit for virus delivery. I use BitDefender (paid) and AVG (free) for virus protection and Ad-Aware (free) for spyware removal.