T.C.
It can be hereditary. I had a baby tooth that wouldn't fall out. I was 16. so the dentist pulled it. I had braces, and so he cut into my gum, attached a bracket, added wires (attached to the rest of my braces), stitched it back up, and pulled my tooth down overtime. It looked a little weird to only see wires going up into my gums attached to a tooth we couldn't see.
Sounds nice, eh? Well, it wasn't! He killed my tooth doing it that way. He pulled my tooth down so fast, it severed the root and now I have a dead, yellow tooth there. And, I have nice teeth. It's very frustrating.
My dad (and several sisters) has similar issues with his tooth not falling out. His didn't fall out until he was 19 years old. I would have much rather waited until I was 19 for the tooth to fall out and then to be ready to grow in on it's own. If I could redo it, I would definitely NOT allow them to pull my tooth down. Oh, and when he cut into my gum to attach the bracket/wire, he cut into the nerve of my tooth next to it. My current dentist told me that it will likely die too. Nice. It really irritates me if I think about it very much.
It could be hereditary somehow. They might have jumped the gun in pulling his teeth before his others were ready to grow in. If they want to do anything similar to what they did to me, I highly recommend NOT doing it. Or make sure they do it very, very, very slowly.
good luck!