Sorry for you hvert. Even infrequent people in our lives can be meaningful.

I don't see any harm in contacting his friends and inquiring about him. Instead of asking for his new contact information (they might be reluctant to give it), you can ask them if they will tell him you are concerned about him and would like to hear from him.

One of the on-line people locator sites might have a new snail mail address for him.