Chickens are social animals by nature. If you leave one by itself, it is misearable. They need the companionship of other chickens. it sounds like you do not have a good rooster that takes good care of his flock, but with only one hen and one rooster, you do not have a really normal situation. He might do better with more hens. He is the dominant one in the small flock and his peckingis a way of showing his dominance. As long as he is not drawing blood with hsi pecks, it is not that unusual behavior. And he is young. He might grow out of it.
The danger you have is the possiblility of a bare backed hen. Sometimes when a rooster mates with a hen, he tears off some of her feathers, either at the back of her head with his beak, which usually is not a real problem, but also with his spurs on her back. If he rips off the feathers on her back, his spurs can then tear her skin, seriously injuring her, possibly fatally.
This does not happen all the time. It depends on the size of the chickens (a big rooster with a smaller hen is more of a danger), how long the rooster's spurs are, how rough he is, and how often he mates with her. With only one hen with one rooster, overmating is a real possibility but it is not a sure thing. Some people have one rooster and one hen and do not have this problem. There was a recent post where someone had one rooster with eighteen hens and some were barebacked. Obviously the more hens around the better the situation, but there is no magic number.
If a hen loses a feather now and then during mating, no big deal. If bare spots start to develop, it has become serious and you need to do something. You can get a saddle for the hen that protects her from the rooster's spurs. You can trim the rooster's spurs so they are not as sharp. You could get more hens. But with only one rooster and one hen, separating them would not be a good situation.
