Did your Vet do a skin culture? If not have him do one or find a Vet that will.
Now, things not stated.
How old is the snake?
How long have you had him?
Did you feed live?
Type of cage an bedding?
Was he housed alone?
Heat source?
Last time you handled him before the problem what where you doing handling?
Any chemical room cleaning or bug treatments?
Any reptile zoos near you or Vet schools? Might help for an expert to take a look.
Good luck!