How old is the rooster? If he’s also an adult, I’d give serious consideration to removing him from the flock.
ETA - have you seen him causing the missing feathers? I don’t know anything about feather mites, but wondering if those could be a possibility.
I second this. If your hens aren’t of laying age yet, they are not ready to accept the advances of your “rooster”. If they’re all the same age and hatched this year, they are pullets and a cockerel. He will sexually mature much faster than they will, so he will be wanting to mate, and they will...