If at all possible:
Find his password.
Change it.
Don't let him find it out.
CHANGE THE PARENTAL CONTROLS.

Think of it like an addicted drinker; he's lost his ability to say no. I play WoW and I am not/don't think I'm addicted.
Another idea:
Delete the financial information on the website.
Max out/Cancel the credit card he's using to pay.
Cancel his subscription.

He's got to reach level 70 at some point, but then the expansion will leave you in shambles again.
As for checking his websites, he's probably got it automatically erasing, but look for the "history" tab on the browser. Good luck.