how long have you had him, most livebearers live about 1 1/2 years, by the time you got it it was probably close to 6 months old. Have you checked the ammonia/nitrite/nitrate readings?
18 - 24 months is the normal life span of most livebearers and ive recently lost most of my stock of Swordtails Platties etc etc as they were at least 12 months old when i got them with a secondhand setup