And a friend has some of the same family of chickens and don't have this problem---------Yet they seem to be healthy other wise and lay eggs, what could be causing this?
Well I would guess it is him causing it, since they are missing the feathers and he is not. I've never dealt with this so I don't know what to suggest. I know they make saddles for hens to protect their backs during mating, or you could maybe switch him out for a gentler rooster.
The saddles or aprons appear very easy to make. In your situation, I would think it would be worth trying to allow the girls time for thier feathers to grow back. This is a link with some info. https://www.backyardchickens.com/web/viewblog.php?id=4031-Patterns Maybe you could swap out your rooster for a smaller one to someone that has larger hens?