Wow. WOw. OUCH. Can't imagine any kind of rock that would do that.

I have never seen anything like that before. Wow. I would get him into a bucket with very clean water and perhaps put some antibiotic ointment on that. More than anything I would get to a vet. That is massive tissue loss. The water parameters will need to pristine for that frogger to have a chance. You could try doing a bath with lamisil but I'm not sure what you need.

If you do try spreading neosporin on it make sure it is plain ol' Neosporin NOT the stuff with pain meds.