I have one African Grey who I continue to battle the plucking problem with. He was seized in a "breeder gone bad" situation. When I got him(her?) He was practically bald. At least now he has some feathers regrowing. Now if I could just get him to leave the rest of his feathers alone...
if you have not already, you should try giving him some red palm oil. it has helped my birds a lot you will see a huge difference in his feathers and skin