It sounds like he had IPR and had a wire tie applied:
If you ever use wire tiebacks with your orthodontic wires, here's a tip that can save you and your patients some time. If you have a single wire tieback and it breaks, you may have a space or two open somewhere. You then have to spend one or two appointments reclosing the space. All you have to do to prevent this is to use two wire ties for the same tieback. You just use two ligature ties held together while doing the tieback.
For some reason they won't break!
From here:
Tips on delivery systems, wire tiebacks, and tricky ligatures | June 2007 | Orthodontic Products
Wire ties, it's my understanding, are used to apply additional force to certain teeth (to get them to move properly).
Or it could be a laceback, which you can see a pic of here (scroll down a bit):
Photo Gallery
What sort of retainer did your son have? If he had an essex (clear tray, looks like a tooth whitening tray, or invisalign) it could have become warped due to heat, or just lost it's effectiveness after such a long time frame passed. There is some debate about how long they last and if they are good for the long-term. If he had a hawley retainer (traditional, with a wire that runs across his teeth), the wire could have bent, causing his teeth to shift. Also, our teeth move throughout our lives, and having braces off at 14 could mean the filaments had not "set" and really anchored his teeth into place. Hopefully this time, since he's older, they will.
I'm probably not supposed to plug other boards, but, the link I gave you with the pic is a really great orthodontic site for adults and older teens. I don't mean to detract from this board, but it's sole focus is orthodontics and there is a lot of really great information on there. You or your son might want to check it out.