Balding hens

I'm a relative newby (got my first batch of chicks this last spring) and have learned a lot from the forums here. I have a question about my hens who are all bald to varying degrees right in front of their tail feathers. At first I thought it was molting, but it's just in that one spot and the new feathers aren't coming in, so my father-in-law suggested it might be mites/lice. But wouldn't that affect them all over, not just on their backs?

This forum entry has some good info -- https://www.backyardchickens.com/t/600698/bald-spots-on-the-backs-of-hens-near-the-tail-feathers and got me to thinking about it... I've noticed that when I do a head count before I close the coop at night, the rooster is always sitting on top of one of the hens, straddling it. I don't know if this is normal or not, but I'm wandering if this could be the cause. If so, how do I stop him?

Any hints or suggestions would be greatly appreciated.