It sounds like it might be spreading. It seems the bump will enlarge and become infected until a parasite bursts out. Which gives you a combo of problems both bacterial and parasitic. Methylene blue and Maroxy are heavy anti bacterial meds, maybe you should grab something like that and at least attempt to treat him. If you can get a powder form of meth blue, you can actually dab it directly on his head with a qtip, it might be tricky. Other than that, maybe a small dosage of a copper based med for the parasites, cross your fingers and say a prayer. Don't be surprised if he goes quickly though It sounds and looks like it might be in the final stages.