I got letters (dead tree version) from strangers when I was in Korea. It was a good assignment, but it was a different country with different customs, and it was good to hear from folk from my 'home' country.

It's comforting to hear about small things: the weather and what the winter's like, local happenings - the usual small-talk subjects. Ask him how he's doing. All you're really doing is reaching out a hand, and not everything needs to be in a first message. He may not feel like corresponding, but if he does, you'll get an idea of what to write about.