The hoof is much more than just a nail. It's live tissue inside with both soft tissue and bone.
When there is an inflamation like he had, the outer hoof wall will become distorted, or crack and fall off. I think he had some of his come off. The wall can be repaired with a bondo-like substance ~if~ the source of infection and inflamation has passed, but the new hoof will still take about a year to grow down from the hairline to the ground.