Has he been like that in previous summers? I have a (maybe weird) theory about teens, which is that lots of them, especially introverted ones, need a 'chrysalis summer' where they see no one, do nothing, as they make that transition from childhood to adolescence. I had one. Both my DC did. It's important not to let it become a habit they can't break out of.
But I'd indulge it a bit, while ensuring he still develops in some way, on his own, not just vegetates. I used to chat to DC at that age about their dreams for the future, and then gently suggest things that would help towards those goals. DS1 got a set of weights and worked out in his room. DS2 got a pull up bar and learned to swim long distance. I took them to a shopping centre, gave them money to buy a new outfit each. Took them to bookshops and libraries and got them to choose a couple of books that interested them.
But they are still our children, under our care at that age, so I did encourage them to do one summer club - even if just for a week - and said if they didn't choose one, I'd choose one for them, just to get out of the house. DS2 ended up loving a music camp as he got some really cool teachers who he wanted to be like.
If he has a family holiday, a summer camp - even just a weekend one, to develop something that interests him, and a couple of personal projects, that's fine.