It looks to be just some sort of genetic mutation, nothing to worry about...just something extra to make him unique.
I would, however, be concerned about those rocks...they look to be the same size as his head or a little smaller...they could prove to be a swallowing hazard especially as he gets older/larger...
In the immediate, they may prove to be a bit of a housekeeping nightmare for you...
These type of rocks hold lots of gunk in the gaps and despite your best efforts, there will still be loads of gunk down in them...I had something similar...I was shocked when I took them out! I now have sand which I like much, much better...