two things actually...pretty strange, I had a dream last night that I ripped my fingernail off and there was another one under it! Real soft, but it was there!
On a serious note, my dad has always had his big toenail fall off. Every few years, it just falls off as the new one grows under it. My sister and I always thought it was the coolest! Depending on how old your son is, it may need to be covered just so he does not pick at it. The nail will grow under the old one. It will push up on the old one and just detatch it! One day the old one will just fall off! If it's pulled though, it will hurt because it may not be ready quite yet.
Hope that's helpful!