It could be an infection. Do you have a trusted reptile vet near you?
Usually when you see that injury in that location it's because the animal opened his mouth too wide - it was forced open whether by falling or whatever (they climb the wall and accidently slip down, catching the beak on the wall which forces the mouth open too wide) - and it is now infected. It will have to be cleaned out by a vet and he'll need antibiotics. In the meantime, you should cut his food up into small pieces so that he doesn't need to open is mouth wide to bite off pieces.
@Yvonne G can help.