How old are they? Those behaviors sounds like they are pretty mature but that type of feather loss is more common with immature cockerels and pullets. It is possible his technique will improve with time if that is the problem.
How much difference in size is there between him and the girls? It's typical for a rooster to be heavier than the hens of the same breed but the more difference in weight the more likely this type of problem.
To me it doesn't sound like either of these but I'll mention them as possible. It is perfectly normal for an occasional feather to fly during mating. That is not a problem. It's when bare spots develop where his claws are sitting or on the back of her head where he grabs during mating that it can be a problem. The head grab is a natural part of mating, it is the hen's signal to raise her tail out of the way so he can hit the target. There would be no fertile eggs without the head grab. Bare spots can be cut by claws, spurs if he has them, or by the beak. I agree with Aart, it probably bothers you more than the chickens. But if you are seeing bare spots you need to watch for wounds. Wounds can happen.
If the entire feather is gone it should soon grow back. But if any part of the shaft is still there that feather will not grow back until they molt.
So what can you do? As Aart said, remove the male from the flock, either for a time or permanently. Or dull the sharp points of his claws. There are different techniques for that. We can discuss it if you want to.
Aart mentioned "weak" feathers. Weak or brittle feathers can be caused by heredity or by diet. Seven out of eleven sounds like a lot for it to be heredity but you never know. Seven out of eleven sounds like it may not totally be diet either. I don't know what you are feeding them but you might try giving them a few Black Oil Sunflower Seeds (BOSS). These are loaded with oil so you don't want to overdo it, just a few are enough. The oil can lubricate the feathers, softening them and making them shiny.
You can continue as you are, just observe them and look for wounds. Often this is enough.
I think this is way too drastic for you. When I started my flock I had one boy and 8 girls that were approaching a year old. Two of the girls developed fairly severe bare backs. I wanted to hatch chicks so I needed a male and it looked like brittle feathers to me. So I ate those two girls. None of the other girls developed bare backs even though my girls went from 8 to 6 so I couldn't blame the boy. You often see male to female ratios mentioned for this, I don't believe that at all. You meet those ratio requirements with eleven and are still having issues. None of the pullets I hatched from the remaining hens developed bare backs, I think it was brittle feathers and possibly hereditary.
Good luck!
