He's only young, so he may settle down, though it sounds like he's really rough. Over time he should start trying to mate with other hens. But your little one may need separating, and if you have to separate her she may never be accepted back into the flock and may suffer a lifetime of ostracizing, which could mean you basically lose your flock's equilibrium regarding her, all because of a silly roo. If she is more valuable to you, and separating her is the only option you can see asides from getting rid of him, then get rid of him.
In my experience, a decent roo won't abuse a hen or pluck her, even when he's just young and experiencing all those 'raging hormones' people quote as an excuse for abuse. Clumsiness is tolerable and normal, but continual damage to the hen, and continual ignoring of her rejections, is abusive and not natural. If he doesn't grow out of it I'd remove him for her sake. We've altered and bred in and out all manner of instincts in them, and other domestic animals, which negates the common and erroneous view that if an animal does it, it means whatever they did is 'natural' and correct for their species. It's akin to 'bulling' in cattle, where some young animals in the steer pen will pick a victim and mount him repeatedly, and all the others will join in, until he dies, unless he's separated first. This is not natural by any stretch of the imagination, but a byproduct of human breeding and management of the species.
In my experience it is not the number of hens that makes a rooster treat them well; you will often hear of a rooster who lives with a hundred hens, or more than he can actually 'cover' anyway, who nevertheless picks on one single hen. It's not the number of hens that makes a good rooster. It's nothing to do with the hens.
Also, all a rooster really 'needs' is one hen, and if he can't treat her well he doesn't deserve more. He'd have to be carrying some incredibly valuable genes to be worthy of an exception there. Any more than one hen is a bonus, not some kind of basic need. Some roos even feel stressed when they have more hens to look after, especially those with better instincts. They can measure food supplies etc at a glance, which is why some roos starve while feeding hens, when not enough food is given; hens will do the same for chicks; this is one reason why an intelligent roo trying to look after too many hens will be stressed by it.
Generally speaking, the more stupid he is, the more abusive he is, but the more suited to a massive harem he is, too, as all he's good for is mating and then being turned into dinner. He's not concerned about looking after them, so being overcrowded by stressed hens doesn't bother him in the slightest; obviously this sort of rooster is ideal for some intensive commercial operations. That sort of rooster, if you choose to keep it, is best suited to being the sole rooster in large flocks. I don't like that sort. Too stupid for their own good, too much bother, and too damaging to their flockmates. Often they're human aggressive too.
Giving abusive roos more hens doesn't make them not abusive as they usually simply continue to pick on their 'favorite'. Some will stop though. It's worth a try if you really like him and don't mind the rough treatment of the hens. If he's not abusive but is making a pest of himself with his attempts to mate often, then by all means supply more hens. Some hens are not interested in roosters and will never invite a mating or participate without complaint. I found with my hens they liked to mate approximately as often as the roosters, and being treated well was apparently quite the aphrodisiac. Not being harmed during mating also meant they didn't start avoiding the roosters, too, so it worked for all of them. Young cockerels just into puberty never had to fight or chase to get hens, because they had instincts about being good mates that the hens apparently found irresistible. Hens like roos that look after them, generally. Some high production hens, or those raised without roosters, can never find an appreciation of having a mate though. Sometimes it can take a generation or two of rearing them with a family unit before their offspring begin to act naturally. Roosters raised without hens can also be socially inept to put it nicely. This may fix up over a generation or two, is what I'm getting at there, but your hen may not last that long and there's so many great roosters you can keep instead of abusive ones.
He can understand she's not keen and is protesting, and that he's hurting her when he rips out feathers, they're not that stupid, but some just don't care. A good roo looks after hens and doesn't hurt them, and part of that is being intelligent enough to back off when she lets him know she doesn't want to mate right now.
I value that trait because it ensures that roosters are able to be let to run with hens and babies throughout their lifetimes, because if any of the hens are injured, laying, brooding, sick, dustbathing, drinking or any other case wherein a rooster should not jump onto a hen, the hen will simply not get jumped on. They are safe even when injured and among the whole flock. This respectful trait is a lifesaver and protects my hens and this gives them quality of life too. Way too many people only consider the rooster's quality of life and totally disregard the hens'. Their interaction with and treatment of the other gender is a heritable trait and can be bred in and out. If you get a roo who abuses hens, and you breed him, chances are most if not all of his sons will be the same or even worse. Just like breeding bullies of any stripe, it's the very same thing, but too many people consider abusive roos 'normal' and 'natural'. In the wild, males that abuse their mates don't get to pass on their genes, lol! It's not natural.
If he's cruel or callous, he isn't worth breeding. Some roos have mating instincts mixed with fighting instincts and routinely damage hens while mating, they're hateful and aggravated as a rule; these are best culled. So if you think his behavior looks like more than just youthful excitement and inexperience, get rid of him before he does away with your hen.
Best wishes.