I re-read your post and saw that he hasn't had access to the internet (as far as you know) for the past year. My mistake.
I would like to reiterate how important it is that you get help for him - immediate help. Not only has he been damaged, but now his sister has also been damaged. And, the damage to children who are exposed to graphic, hard-core p 0 r n is long-term and, often, lifelong.
Your post should be a SERIOUS INDICATION to all parents with young children out there to turn off the computers, turn off the videos, have a care as to what you allow into the home, and always, always, always supervise the children, EVEN if it is inconvenient!