You are not alone! Similar situation here - we went away for a few days when they broke up but we've been back for over a week and DS14 is barely leaving his room, which smells, as does he (unless I threaten to turn the internet off and he showers). Gaming & watching box sets - that's it. At that age I was out every chance I got - but it was a different era & home was pretty dull. At that age being 'grounded' was the worst punishment imaginable - us telling him he can't stay in doing this the whole time unless he goes out sometimes is ironic!
We're negotiating at the moment - I want him out of the house once a day - ideally the gym, see a friend, but even walking round to the bloody corner shop will do - something, anything so that his muscles don't atrophy and his internal organs keep on functioning. I would NEVER have imagined this of my smiley, sociable, sporty, outdoorsy pre-teen - but now he is this tall, skinny, taciturn, deep voiced man/child (with the odd flash of my boy, every now and then - also thankfully still polite and sociable, to the minimum requirements, with family and visitors, and the odd chore, but that's it & I tell him I love him every day - get a gruff 'love you too Mum' back sometimes too if I'm lucky!).
There is also the fear of mental health associated with this reclusive behaviour - I've read a lot about the risks of teenage boys suffering and lighthearted comments aside I think there has to be a level of awareness - I know a friend of a friend who didn't see the signs in her boy and had the worst outcome imaginable - I don't know what the answer to that is or how you know when it's not just 'down time' but something more? Good advice/warning there from Rideon - I am thinking we need a bit of a plan..
It seems adolescence can be painful for everyone, at least that is my current experience - and until you've seen it from both sides (as a teen yourself back in the day, then as a parent of a teen) you don't understand. I'm also sending in fresh fruit and trying, with various degrees of success, to make him sit in the sunshine for 10-15 minutes a day so he doesn't get rickets or scurvy!