Is he plucking himself to eat the feathers or just eating feathers floating around? Feathers are almost pure protein. It’s normal for them to eat feathers just floating around. Chickens go through two full juvenile molts before they reach adulthood. They outgrow their feathers and have to replace them. Is he just molting and eating loose feathers instead of plucking feathers to eat?
If he is plucking his feathers check him for mites or lice.
Poop is rich in partially digested nutrients. Eating flock poop is how chickens spread probiotics and develop flock immunities. It’s perfectly natural. If the rest of them are not eating some poop, then that behavior is unnatural.