How difficult and worrying for you - I gather you are not in a position to drive up there and knock on the door?
I was just having a similar conversation with a friend, saying how I wished next of kin would go through the address book and let people know.
My suggestion is (apologies if you have done this already):
Write John a letter, and indicate that you wish he or someone caring for him would let you know his situation. Give as many contacts (email, phone etc) as you can, and enclosed a stamped addressed envelope.
Address it to John Smith (or anyone concerned with his care) so that anyone can open it.
On the back of the envelope write you address so it can be returned.
Anyone popping in (even an estate agent) can deal with that. You may still not hear back, but I think that makes it most likely that someone will let you know.
I do feel for you, I have kept up with some of my parents' friends, and find contact dropping off, but they are not people I am immediately so concerned about. I do hope you hear.