Being able to see that much down and potentially skin isn’t normal, so it looks to me like it’s more than just a bad molt (though a bad molt can instigate feather picking). All sorts of things can cause birds to engage in feather destructive behaviors, from boredom to dietary deficiencies to irritation.
Has he been to an avian vet to rule out any medical conditions? That’s a good place to start, to ensure he isn’t doing it out of pain or discomfort.
Some other questions: What’s his diet like? Is he very active, or is he the type to sit and preen most of the day? Have you noticed any excessive hormonal behavior? What’s the humidity like in your house (if you have a hygrometer)?
Wishing you luck.
