I have had him for a little over 2 yrs. He was almost full grown when I got him. I have been using salt treatment. It has kept it from looking infected like it did in the beginning.3 months is quite a long time. Sometimes the body "walls off" cysts or "pockets" of infection. They may work their way to the surface & out. I'll ask too, how old is the fish? How long have you had it? As fish age all kinds of "stuff" can happen...everything dies eventually from "something".
The fish is eating & active. I think I'd go with lots of water changes & maybe a low level salt treatment, if all your fish are salt tolerant. At this point they've all been exposed to whatever it is. Keep a close watch on changes & be ready to move it & TX with antibiotics if it seems needed or it's harassed by tankmates.
Good luck, Claire!