Not sure if you are teens or adults, if adults you might have a bit more options.

It must be scary not knowing what is happening to him. What caused his reaction we can't know of course, could be quickly oncoming depression, a psychotic break, something that physically happened inside his head, some accident or abuse he just had to endure, a trigger that caused memory of past trauma pop up etc. What I mean is "something" happened... and it must be extra scary not even knowing what it IS...

Because of this you don't know how you could even help. It's too bad he didn't tell you, but I guess he might be too depressed or too afraid...

I understand that you don't have many real options to get through to him because of his parents. But if it was me I would try to get one single message across like "You can talk to me if you want to, I AM here if you need me, I will not intrude anymore but please know I am your friend".

Man I'd go tape that on his window. Just letting things be is the hardest of them all, not something I am good at.

I hope this passes and that he comes out and that he reconnects with you.