Test your water, #1. If there's anything wrong with the water, start to fix that at the rate recommended on the additive bottle/jar, no faster, except by a partial water change or running carbon. Don't feed: it's tempting, but if it is water quality, feeding can only make it worse.
#2. If there's nothing wrong with the water, consider disease as a possibility. Google up his species and see if you can find anything about peculiar ailments for his kind. You may need to put him in a quarantine tank for a treatment if you can determine what that should be.