He's been inside for the last month or two due to cold/wet winter. It must have happened last summer, when he was living in a temporary enclosure that had dirt but also cinder blocks around the perimeter. He does 'dig' a lot, trying to escape any enclosure, so it may have happened then. We are in the process of redoing his habitat, and he will be living in a nice heated shed with a linoleum floor and his outside environment will be the entire backyard, which includes dirt, grass, tile, and pavers.It will callous over and heal in time, but you must figure out the cause and prevent it from doing further damage. Is the tortoise living on concrete?