You could try separating him for part of the day, or put himmin a pen for awhile. How many pullets do you have? Is he favoring the ones with the thinned feathers?
In my first year of chickens, some of my girls got similar bare spots from an over eager young cockerel mating them. The loss of feathers there can take until the first molt is completed for feathers to regrow. I used hen saddles on the ones who were bare. The next year, they were all fine and...