Do you happen to know what the Ammonia, nitrIte and NitrAte is? If his water parameters are off/have been out of safe zone he could be sick from that.
Did he get over fed last week?
Before the recent water change, when was the last water change and how much water?
What size is the tank?
Are you familiar with the nitrification cycle? Did you cycle the tank and how long have you had him, and also the tank?
Do/did you replace the filter pad in the past few weeks /month or so? Or rinse out in faucet water?
In the past month maybe 3 weeks back had there been a long time that he did not get a water change?
It could be a mini cycle if a pad was rinsed in untreated faucet water, or replaced lost the BB to keep it running smoothly.
Could be internal parasites...or constipation...have you noticed the fish pooping recently? look normal or white and stringy?
If it is a constipation issue you can feed him the inside of a cooked to soft texture green pea the round pea inside the outer hull of the round part/actual pea. Since he has been fasted he will probably try and eat it. Try and squeeze some juice from a piece of garlic clove on it if you try a couple times and he still doesn't eat it. The garlic usually makes it more appealing to fish.
Test for Ammonia, Nitrite and Nitrate, - pH, GH and KH as well if you have the test liquids or go to the store and have it tested. Get the actual numbers for each thing from them, not just it's fine or a little low/high, etc.
In general any time I have a fish not looking well there are a number of things to think about, these are some of the very basic ones.
Also do another pwc (partial water change) let's say 50%. Make sure that the dechlorinator you have is being used at the correct amount and neutralizes/binds ammonia AND Chloramines if your water supplier uses them. Usually it will tell you. Chloramines are tougher to break down and last longer to keep out tap water "safe" for drinking and toxic to BB (beneficial Bacteria) which keep your tank able to process waste in the nitrification cycle.