HI
As Cindy mentioned, this is part of their natural grooming. They shed the "sheath" on their nails, that is how they keep them sharp. They can scratch things with the frontpaws but sometimes need to pull on their back claws. (As a side note, I think that old cats always have the super thick back nails, and sometimes front nails, because their arthritis keeps them from doing this bit of grooming.) Trimming his nails in the back will make the excess layers come off easily.