I don't fully know how to cure it, but so far, I think your doing good. Just make sure he has clean water (No slacking in watechanges) and everytime you do a waterchange, add 1/4 to 1/2 teaspoon of aquarium salt per gallon of water.
hey, crown tail, one ray starting to grow funges and climbing. started salt bath in 1 gal today. started when I put him in a new 1.5 gal, filtered with a Nano filter and baffle( pic in profile) help bro