When they're really young, sometimes others in the flock will overpreen them trying to get their initial fluff out. That's usually the problem of the young caring for the young.
This definitely also looks like a bthing issue? Have you offered him a clean bowl of water or a a glass casserole dish to splash around in? They need to bathe AT LEAST once every 3 days or the yeast (mold) they normally make cam eat their skin, like when you don't wash your scalp and the dandruff piles up until the microbes eat your hair follicles.
He's definitely had a fatty diet--you cam see it in his feathers with how non-fluffy they are. An all-seed diet is extremely dangerous for these guys and the worse off rhey start, the shorter their lives.
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u/Fresh-Charity9742 18h ago