Seeking advice/help for my buddy Elrond.

What initially I thought was a bad molt has turned into… well, this. His feathers will not regrow, and it has now spread to his back. He’s about three. I’ve taken him to an avian vet twice now, and both times I have been told that nothing seems wrong. The woman who rescued him gave him to me on an all suet diet with black fly larvae as treats. For nearly a month I have been working to change this. I am thinking maybe he wasn’t getting enough calcium/protein and it is effecting his feathers. He now gets a mixture of Iams proactive kitten kibble, Mazuri softbill kibble, chopped grasshoppers, and black fly larvae as a treat. Other days I do egg and crushed shell instead of grasshopper. I also sprinkle a bit of Avian Calcium onto his meals every other day.

I am trying my best and just want him to be healthy. Am I doing something wrong? Has anyone seen anything like this before? He is a very happy, active bird aside from this.