What kind of circulation do you have in your tank. Could be an o2 deficiency.
Any new fish added lately? Could be a newly aquired parasite.
Is HE a new fish??
We will definately need more details about his environment.
Preferably,
tank size
bioload (How many and which kind of fish)
What equipment do you have on/in your tank? (Powerheads, sump, live rock, etc...)