Ok more information for you.... Move him to a hospital tank.
This is a symptom of a becterial infection, or it could be an early sign of flipover disease.
Check your water and make sure you check your nitrates. If readings are normal you should treat with an antibiotic for bacterial infections. You may lose the head-down fish within a week to ten days if a bacterial infection is the cause. Other fish will die later if you do not eliminate any parasites ( shich probably precipitated the infection) and bacterial invasion of the fish...
Hope this helps better... sorry for the grim outlook