Sorry the forum is pretty quiet today.
Poor little fella. I have no experience with H-I-H disease, just read about it while diagnosing other issues.
Tropical fish have pretty different needs than goldfish so I won't offer any advice. Do research and take action but only necessary medications as treating for the wrong ailment will only further stress your fish.
Isolate the fish into a quarantine tank(search or look for stickies about them) and sometimes the best thing to do is very little. Lots I fresh clean water and not much commotion otherwise.
Good luck!! Edit: also if Ich is off the table After you research then perhaps the water temp Gould go back to the normal Goldie range.