A.D.
My kiddos started doing that about that age b/c he was pulling himself up to standing and he didn't really know what his feet were for... he'd roll his toes over and stuff. The nails were so soft still that they'd just bend and split. Once he learned how to walk CORRECTLY and pull himself up CORRECTLY it wasn't an issue and hasn't happened since. Does he pull himself up or stand up on his toes? That could very well be the reason. Just a thot.