Yep, mac blocking. Some newer Linksys routers also have features that only allow net access during certain time blocks. Or if the router is in a specific secure location you can always unplug his ethernet cable from that location and leave it hooked to his pc.
I have to agree with Lexluethar, Just about anything you can and will do he will find a way around if he is a smart kid. Do your neighbors have wireless internet? When I was younger and doing almost the same thing my mom took away the wireless router (i had set it up she didn't know anything about it other than it gave me internet).. Well, my computer picked up and instantly connected to a neighbors router that was the same brand as mine, i didn't even notice it was gone till she walked in my room and was like "How are you on the internet?" All we could do after that was laugh. Netnanny can be gotten around with some tech savvy skills and so can other programs like that.
In my opinion your best bet is either a router that allows time control (mine is a WRT350N) and it has the option though I have never used it, take his computer away completely, or something like his monitor (can't exactly game without a monitor), or the classic butt beatin.
As others have said, not telling you how to raise your child, but my opinions and experiences.