Finally time for a positive update!
Mouth still looks pretty bad (but doesn't seem to be getting worse anymore), two days ago I was on the verge of buying antibiotics, but I wanted to give him one more chance at eating. I went to feed the tank, and he showed a ton of interest in food for the first time in a while, but didn't take any. I then fed bloodworms and he aggressively went after and ate a few! Yesterday he again ate bloodworms, so while its not the most nutritional meal, at least he's getting something in him everyday again! He also seems to have more energy again and isn't hiding a bunch anymore.
This is giving me hope that clean water and salt is working and antibiotics aren't necessary! If he stops eating bloodworms and gets more lethargic again, I'll immediately treat the tank, so fingers crossed he continues to progress in the right direction!