I'm not an expert but I think if you see a fuzzy white patch on him and he's got fin deterioration it sounds like columnaris.
An anti-biotic appropriate for gram negative pathogens should be used from what i've read. And an anti-bacterial food if he's still eating, make sure it's for gram negative organisms. Keep his tank at around 76 or so. Columnaris thrives in warm tanks in the 80s. This is just from the research of done on it.
I've had no experience treating for columnaris.
I hope your little guys makes it!