Have you ever noticed him biting his toenails? My Pom sometimes does this, and she has the same problem - sharding as you describe it. It's like they peel off in layers, and hers get very sharp too - like cat's claws. She hates having her nails cut, but that's the only way to remedy the situation and keep her from biting her toenails, so I have to take her to the vet or groomer to have them cut, since I personally can't do it without coming up on the short end of the stick.
If your dog isn't biting his nails and causing this problem, then I don't have a clue. I hope you can find an answer.