One of those difficult questions to answer, it could be that there may still be a little radiating pain from the original injury, it could be a secondary issue whether stress related from the injury or not. So many factors that can influence feather damage bacteria, yeast, injury, emotional stress, ingesting a foreign substance, heavy metal toxicity, lack of unfiltered Sunlight, something lacking in his diet or too much of some nutrient and the list goes on. It does appear that he may be barbering feathers himself but even a vet visit may not provide a clear answer. Though it would be worthwhile to discuss with your birds Veterinarian.