You don't have a rooster you have a cockerel. Two totally different critters.
What you are describing sounds pretty normal. When the hormones hit the cockerels can go kind of bonkers in mating the pullets and hens, though they are often less successful with mature hens. It's not that they are sex-crazy maniacs trying to fertilize non-existing eggs, but that the mating act is more about dominance at that age. The one on bottom is accepting the dominance of the one on top, either willingly or by force. At that age it is usually by force. Their hormones are telling them to dominate.
When they mature, cockerel and pullets both, they usually change into a pretty peaceful flock. The hormones settle down and they all accept their mature position in the flock. It can be hard to watch them go through that process though and not all mature.
In a free ranging flock with a dominant rooster, mature hens, and younger chickens the adult rooster would run the cockerels out of the flock at this stage, forcing them to survive on their own and see if they can mature into an adult. That's pretty common with many social animals in the animal kingdom. You don't have one of those flocks. The mature hens may control him a bit but probably not much, nothing like a strong Daddy figure would do. Usually Daddy takes better care of his hens than he does of his pullets when it comes to cockerels. After all the hens are the ones laying eggs.
Are the girls bleeding? Are they losing patches of feathers? While it may be hard for you to watch it's probably not nearly as bad for the girls as you imagine. In many respects that's how they work out the social order in chicken society.
You have a few options. As long as you don't see blood or physical damage to the pullets or hens, you can leave them alone and let them work it out. That's what I usually do.
You can isolate that cockerel for a few months until he and the pullets mature. That's what I do if it gets too rough for my tastes but I'm only separating them until the cockerels hit butcher age. In your case you can try putting him back with the flock and see how they behave. Or you can isolate him for the rest of his life.
You can get rid of him. Try to sell him, give him away, eat him, or just kill him and toss the body. Your choice.
Why do you want to keep that cockerel? What are your goals toward him? The only reason you need a rooster is if you want fertile eggs. Everything else is personal preference. Nothing wrong with personal preference, that can be a strong motivator. It's up to you to decide if the correct number of males in your flock is one or zero.
