ouch, maybe try and pull him and dab some methylene blue on the damaged area. being careful to avoid the eyes. this should eliminate the fungus. if any. which would be my main concern. Then, it's all on him and how much he can heal the missing tissue. maybe not a lot, but some. besides keeping an eye out for fungus, not a lot you can do either. maybe softening out the place where he got stuck or making it bigger so he can't do it again. i might add in a few catapa leaves as well. they'll darken your water, but help prevent infections.