It sounds more like you have a cockerel, and pullets, not a rooster, and hens. For a cockerel, and pullets, you've described normal behavior. Regardless of age, and technique, eventually the hen's backs are going to show some wear, and tear. You will need a large cage, or separate enclosure, (a bachelor pad) to put the rooster in, when the hens get nearly bare backed, and when the hens are molting. Once the feathers are back in, you can put the rooster back in with his hens.
I find I have to do this a couple times a year, and during molt. It usually takes about 4 - 6 weeks for the feathers to grow back in, from being bare back. It takes a bit longer when they molt, so make sure the cage, or bachelor pad is big enough that the rooster will be comfortable during that time.