It happened to me, it was only one of my incisors. But I didn't find out until I was 24, I had to have it pulled and an implant put in. I would see what his dentist recommends and go from there. My cousin's son has the same problem and they found out last year when he was 11, I'm not sure what they decided to do. Their dentist said that it sometimes runs in families and she called me and said that it was my fault.
Good luck, it seems to be more common than one might think.
The price he quoted was almost $20,000 to fix them! We said we would just wait!
Count me in the club too. I'm missing one of my molars, I still have the baby tooth (at 41) and every time I've gone to a new dentist, if they suggest pulling it and doing either a bridge or an implant, I run in the other direction. It's cavity free, isn't loose and is just fine, for incisors, it may be a different issue, but unless they're rotting, or affecting his bite, or asthetically he wants to get it fixed and you can afford it, don't let a dentist talk you into it!