So he hasn't actually tried anything in the past two months? What are the numbers on water params, and how often does he test them? Does the fish have tankmates? When a cat stops eating, there's something wrong. Any evidence he ate something in the tank and maybe has a blockage now? The redness sounds bacterial. The head being sunken in is likely due to starvation, because I'm guessing he hasn't undergone actual physical trauma, or you would have mentioned it. Heavy breathing is due to low dissolved oxygen or ammonia, nitrite, nitrate levels being too high. It could also be due to ich, but there would also be external signs. This is actually a small volume of water for this fish, especially if he has tankmates, and from looking at the youtube briefly, it looks like he does.