Why do you want to keep him? What are your goals relative to keeping him? When people talk about making decisions or what to choose it will be different for all of us. We have different goals and different circumstances. Any decisions should be made on your goals, not mine. The only reason you need a rooster is if you want fertile eggs, everything else is personal preference. Nothing wrong with personal preferences. I have a few myself. But those are based on a want, not a need. I generally suggest you keep as few boys as you can and still meet your goals. That's not because you are guaranteed more problems with more boys, just that problems are more likely the more you have. To me, this has nothing to do with ratio.
Overmating is a term that gets tossed around a lot and can mean different things to different people. Sometimes when chickens mate some feathers are lost. That's not a problem at all unless it becomes excessive to the point that that the skin is bare. That may be on the back where his claws rest when he is mating or that may be the back of the head. In the mating act the male grabs the back of the females head. This is her signal to raise her tail up out of the way so he can hit the target. Without the head grab there would be no fertile eggs. Sometimes there are bare spots on the back of the head or, as Shadrach mentioned, he may grab her comb and tear it. Usually these bare spots upset people more than it does the hen but there is a risk that those claws or beak may cause a wound. Chickens can sometimes become cannibals if they see a raw wound or blood so it is something you have to watch for.
Some pullets or hens are more susceptible to feather loss. Some can have brittle feathers that break really easily. Even if the rooster is a perfect gentleman those feathers can break. Some cockerels do not have a good mating technique and during adolescence mating is often by force. Usually when the mature the boys develop better technique and the girls quit resisting so strongly so this gets better, but some never grow up.
Typically when they mature they calm down a lot, but when the pullets and cockerels are immature it can be anything but calm. The hormones "can" hit the boys hard and the girls are still so immature they don't know how to cope. Sometimes this isn't a problem at all but it certainly can be. It looks like yours free range. All that space is a big help, I consider it pretty important. As someone on here once said, watching adolescent boys and girls go through puberty can be hard on the faint of heart. At that age mating has nothing to do with fertilizing eggs, it's about establishing dominance. The one on bottom is accepting the dominance of the one on top, either willingly or by force. At that age it is almost always by force. Since it is by force it can be violent, hence a risk of injury. I've never seen a pullet injured by this, I think having a lot of space helps, but it can sometimes be violent so injury is possible.
You may notice I'm using a lot of weasel words like can, often, sometimes, or usually. That's not because I'm a lawyer, I'm not, but because different things happen. Each year is different. Each brood is different. You can see these behaviors but it does not mean you will.
Sometimes, especially when they are immature, the cockerel will try to mate the pullets so often and they really don't want him to that they spend most of the day in the coop when he is in the run. They may even spend most of the day on the roosts to avoid him. I hardly ever see this but it's more common if room is tight. I have multiple food and water stations so the pullets can get food and water. This stage usually doesn't last for long and doesn't hurt them but it can be hard for people to watch.
To me those are the potential issues. Reading this forum you'd think all this is guaranteed each and every time. If that were true chickens would be extinct. These things can happen and an injury can be serious, so pay attention. But I totally agree with Shadrach, go by what you see instead of what some stranger over the internet like me tells you will absolutely happen. If your goals don't include having a rooster getting rid of him might potentially simplify your life, especially before adolescence hits. But adolescence may be no big deal for you.
Quite a change for Spain where you were looking after multiple roosters with very small hen to rooster rations. You were always good at observing, I'm sure you are enjoying this opportunity.
Best of luck cheers 
