Scarab, Butterbur, my son (15) is the same, and I don't have any answers. The only thing I can say is that my older son (30)also went through a phase like this. He also went to ground for a while, just staying in his room on his computer.
At the time, I wasn't too worried. At least I knew where he was, and there were some very bad boys on our street (Manchester council estate). Looking back, I think he may even have been a little scared himself of the social groups (no gangs then, luckily) that were always changing, and being with other young people can be very scary! I think he was taking refuge, in a way.
I didn't realise it at the time but my son was learning a lot while he was spending time on his computer. He went on to do Computer Studies at uni and now has a great job, wife, child and another on the way.
I suppose in a rambling way I am saying that the time these boys spend on their computers need not be totally wasted. They do read stuff online, and they do need time to chill. Getting out of sync is something everyone likes doing from time to time, and the holidays are only 6 weeks - though that seems like a lifetime to us.
They'll be so bored that when they go back to school they'll be glad to have something to do!
Not much help, but good luck!