it could be stress from the pecking order in the group (he could be being picked on).
1.Have you noticed him being nipped by the other fish?
2. What color is his poop? If it's long and stringy and white, there's a problem, but if its solid and brown, nevermind this question.
3. It would also help to know the
ammonia level in your tank.
4. Could we possibly get a picture? If he doesn't want his picture taken, don't stress him out more, leave out the picture.
At this point, keeping up with water changes once every 1-2 weeks and watching out for signs of aggression and poor water quality is all we could do at this point. I hope he gets better.