Why does my macaw have scabs? What causes bird skin issues?

This may be due to a nutritional deficiency or infection (parasites, bacterial, fungal). Ideally, Scooby should be examined by an experienced avian veterinarian for a diagnosis. He can have about a dozen macadamia nuts daily, but you can mix it up and give a few Brazil nuts or hazelnuts as well. Almonds and pistachios are also healthy options. Try to get him to eat as much of the commercial pelleted diet as you can. It may be difficult transitioning him but it will be healthier in the long run.
You can also bathe or mist him once or twice a week to encourage preening and to promote healthy skin and plumage.
Clean and sanitize his cage. bowls, toys and perches. Discard anything that cannot be sanitized. Clean the cage bottom daily to prevent attracting pests.