This sounds like a good ol' classic case of Chat Addiction. I remember when I used to be just like that, as an ex-nethead I would cut myself off from other people unintetionally but this would alienate my friends and family and it started to show.
If he is anything like me then he'll grow out of it, but there are a couple of approaches that you can take. You can either use the 'guilt-trip' method that involves emphasising to him how it makes you feel and showing him directly how life is without his presence in the household, OR you can bluntly deny him internet access which is dangerous, especially if he's addicted to it.
If he is addicted to it, it might be worth talking to him in a non-confrontational way (easier said then done though) about why he sits on the computer for hours on end, there might be reasons that he does this, even reasons he might not fully understand.
Though I can't be of much help as it sounds like his behaviour is addictive, here are some links:
http://www.geocities.com/rhema6/chatsafe.html
http://www.netaddiction.com/clinic.htm
Or Google it under Internet Addiction Disorder.
Hope this is helpful!
