Does he have any lesions or bumps on his body? If so, it could be fish tuberclurosis; although, TB is pretty rare.
It could also be genetics or internal parasites, if it were to be internal parasites, then you would see stringy white poo, or even tiny red worms at the business end when the fish isn't moving.
I would try treating him with some AQ, and possibly isolate him in a floating container with different water, since fish that tend to shoal don't like to be alone, but make sure his water is separate from the others. If it does turn out to be TB, (rare) it is best to isolate him. Although, he would be in a lot of pain and you may want to consider euthanizing him ( TB is rare though, and is most likely not be the case)
What helped with my Betta when she had internal parasites was soaking her food in garlic juice. I hope this helped!
Good Luck! I hope your little guy gets better! :)