Be frank, tell it like it is, exactly how you told us. I don't hold grudges and heal them with others quite easily. In fact, you probably don't have to say much of anything unless the other person desires a grudge (as some do - the feeling that someone else has done something you feel is so negative as to sever all contact is a rather powerful and egotistical feeling).
I think you'll find that your friend wants to check in with you as much as you want to check in with them, especially if you had a friendship with them for a long time.
Still lost? Here's a quick guide:
1. CALL HIM!
2. If he sounds happy to hear from you, see A. If he doesn't sound happy to hear from you, see B.
-A. Talk about how things have been, maybe invite him and his g/f to dinner or something. Even if you never see him again, leaving on a good note is healthier and feels better for both of you. Yearly calls are also very friendly and nice, if you choose to care that much. An apology most likely isn't necessary at all.
-B. After talking a little bit, apologize. If his reaction is favorable, see A. If his reaction is unfavorable, wish him a good life and move on with your own, knowing that you did everything you could.