It is time for a veterinarian visit for a wellness exam. Feather quality will defiantly be related to diet. However it can sometimes be from other illnesses that can't be determined without a few tests.
Destruction can also be from emotional stress, diet would be the easiest fix!
While you wait for your vet visit you should try offering some veggies and fruits. You may find he likes a wide range of things. IRN can be good eaters when they have a chance to test things. If a new veggie or fruit scares him just place it in his bowl and he might come up with the courage. If not don't give up and keep offering it everyday. He may try it again and realise it is actually very tasty
Can you send a picture of the cage set up and a photo of the daily diet? It will help us help you. We can only help in husbandry, ideas for your enclosure and ideas on enrichment or diet. You will need to go to a vet to determine any underlying health issues.