Can you please post your water parameters for ammonia, nitrite, nitrate, ph, kh, SG, and temp? How long have you had this fish? What else is in with him?
I have the same issue my larger Occelaris..
Amm - 0
Nitrite - 0
Nitrate - 5
Ph - 8.2
Cal - 400
Phos - 0
SG - 1.026
I have had the clown for about 3 weeks. Was eating very well. Still eating some, but mouth open all the time.