There isn't always a scab or sore. Just like with cocci, there are many different types, and not all have the classic symptom of bloody feces.
Sometimes that classic bumblefoot symptom never occurs, but the staph infection spreads to the leg's bone or other parts of the bird's system.
I don't think that's what he's got, though, it was a suggestion of a possibility. I do believe he has some kind of strong infective bacteria which is most likely highly resistant if not immune to man made antibiotics. (Otherwise it'd be gone already). It could have started with bumblefoot, but there's so many things that could be causing it.
What if he's found some species of mites, like those responsible for mange, somewhere and they've inhabited his follicles? Or it could even be something entirely new.
Anyway, I hope you find what works. If the pharmaceutical treatments keep failing, natural ones should do it. Best wishes to you and your flock.