Does he have access to direct sunlight (not through a window)? He might have a calcium deficiency and this will get worse. Indoor birds need direct sunlight (Vit D3) to help the body absorb calcium. If you can't get hold of liquid calcium (with added Vit D3), you can use human calcium. Unfortunately I don't know the dosage. Otherwise, direct sunlight even if it's only an hour every day, will help.