If you are close and you've never really caught up outside of work, now is the time. I'd recommend a weekly plan rather than ad hoc. You've already got a friendship due to location. Now that location is gone, it's easy to let it slip away.
If he was part of what made that workplace bearablez it can be a wake up call tondo the same yourself and thank him for it. However, I'd be a bit miffed of they left and didn't tell me when they were applying etc.