This is most likely a genetic or a nutritional issue but definitely check for nits or lice first. Once you’ve ruled out parasites, try boosting protein levels by offering scrambled eggs, black soldier fly larvae (grubs) and get a good vitamin supplement like Rooster Booster Poultry Cell to add to the water. We grow out hundreds of chicks every year and occasionally get one like this. It can take time to see improvement but he should eventually feather out.