Here you go, it shows the progression from small white dots, to a large lump at the base of the fish, to the hole. Metro is short for metronidazole and is a treatment for internal parasites it can be used by mixing with food or by disolving in to the tank water . Have you noticed any other symptoms, going off food/spitting food, dark colour, isolating itself from the other fish?
The good thing was he has fully recovered and the wound healed over completely, has a little scar but otherwise is completely fine