First, you're sure this is not just "over-roostering" damage? Is the feather loss *everywhere*, or just on the hens' back/head/upper neck? If that's what it is (and even with 15 hens to 1 rooster, it can certainly happen, with a less-than-graceful rooster or one with certain favorite hens) your only solutions are probably to limit the rooster's time with the hens, or apply saddles to the barebacked individuals.
If it *is* feather plucking, i.e includes tailfeathers and feathers on surfaces that the roo does not come in contact with during mating, then I'd suggest your best bet is a threefold approach: provide a lot more space per chicken (either by increasing run/coop space, or reducing number of chickens), give them something constructive to peck at and sort thru to occupy their time (like a whole buncha produce scraps, or some scratch tossed amongst a pile of straw), and temporarily 'up' their protein (options include black oil sunflower seeds, preferably hullless; cooked meat scraps; catfood, although there are some questions about its appropriateness especially if used in large amounts or longterm; or anything else that will boost protein in an amino-acid-balanced way)
Good luck,
Pat