Geeze, could be a few things....
1) Just plain old...My old fellas stopped eating about a week before passing away
2) Radical change in diet (probably the culprit)
3) BBS Baby brine shrimp fed as a youngster...Some breeders swear by it, others swear it causes swim bladder probs...
4) Low temp in tank ( can affect digestion and cause probs)
Check your tank temp...If it is between 72 and 80 you are ok...Do a massive water change and starve him for a few days...Sounds bad, but they can go atleast 6 weeks before starving to death.
Hope it works out for you