Really hard to tell from that pic. Like h2so4hurts said, if the trigger has small white dots all over his body then it's probably Crypto or Velvet. The best treatment for those two ailments would be a copper based product, such as Cupramine or Coppersafe.
However, if it looks more like white splotches (almost like his skin is peeling off) and not tiny dots, I would suspect Brook. Best treatment for that is a Formaldehyde based product, such as Formalin-MS. Quick Cure also works in a pinch and is usually more widely available (Petco, Petsmart, even Walmart).