Before I met DW I was in a relationship for 3+ years. We were very serious, and were actively planning a future together. We had some communications breakdowns that ultimately led us to split up. The thing was we never met to have final closure on the relationship.
Because of that I used to have a "what if" in the back of my mind. I know she's on Facebook, and she knows I'm on there. She's married with kids, I'm married with kids. Neither of us try to contact each other. There is really no need to at this point. We've both built happy and healthy lives without each other, and I think there would be little to gain if we spoke again. At the time we thought the other was "the one," but that obviously wasn't true.
I've learned that any "feelings" you have towards ex's are nothing but fond memories. A lot of times those fond memories lead people to do dumb, impulsive, things. To the OP, I would unfriend this guy and keep moving forward in your life. Past chapters in life close so that the new ones can open. It's not worth "re-reading" a chapter that could alter the happiness you've created.