I agree that real life friends are needed to help him out of this. I had that internet addiction, still do actually, only just starting to get out of it as my friend likes to drag me out everyday when I would rather stay home on my computer.

When you guys go to the store try to get him to come. Try to get him out and keep him out of the house as much as possible. Take lots of walks and just go out, not meaning you have to do stuff and buy things, but maybe visit a library or a local community centre and enroll him in some activities.

If all else fails, it's time to limit the internet use. Put passwords on the computer, time limits, etc. take the internet box with you when you leave the house so he has no choice but to go something other than the computer, most likely go out with you.

Best of luck!