@Indefinite_Article I would suggest asking in a way that gives your neighbour a lot of latitude to answer however he sees fit.
When it comes up, in the context of regular greetings and small chat, etc., it can be as easy as "How's _____?"
If he wants to answer "Fine," that's okay. If he wants to go further, then he can make that choice too.
Asking at least opens the door to him not feeling isolated in this.