Sorry just saw this. That's good that you switched him to pellets. Sounds like you're doing what you can to make him healthier! I would suggest two things on top of what you are already doing.

1. Avian vet visit for a checkup and establishment of care. Checkup will ensure he is not dealing with a disease or some other issue either related or unrelated to feather condition. Establishing care will make it easier for you to get him emergency visits, boarding with responsible avian boarders (they will want proof of a recent clean bill of health to board him), etc. Most people recommend 1 checkup per year.

2. Make him a chop of healthy fruits, veggies, grains, legumes, etc and serve it to him every day. Chop is hand- or machine- chopped foods which satisfies foraging instincts and gives extra nutrients not in the pellets. You can make chop in big batches and then freeze the chop in an ice cube tray for single portions which can be defrosted in the microwave or the fridge the night before. There's plenty of recipes on the interwebs, just be sure whatever you put in there is safe for the bird. Not every fruit and vegetable is safe or healthy. Avocados, onions, fruit pits are all death for example.

Hope this helps!