This is generally the result of a bacteria infection. Often caused by over feeding, and/or poor water quality. You need to keep his water clean, add 1 teaspoon of table per 2.5 gallons, and use an antibotic.
As popeye is often a secondary infection cause by other far worse conditions. Popeye often leads to dropsy. I'd really reccommend using a broadspectrum antibotic.
Reccomended treatment
-Gel-Tek Ultra Cure BX (trisulfa, and nitrofurazone)
-Any Trisulfa medication (IE Mardel Trisulfa)
-Maracyn Plus
-The combination of Maracyn-TC and Maracyn
-The combination of Maracyn2 and Maracyn