So many possible reasons!
Diet related (lack of nutrients can cause poor feather growth)
Boredom related feather destruction (what is the cage size, what toys are available to him)
Psychological related feather destruction(loneliness, stress)
Health related feather destruction (some birds pluck when they are in pain)
Disease (PBFD, etc)
Genetic (some birds have weird feather mutations)
So what is this bird's diet and lifestyle like? Has he been to the vet to rule out any health problems?