I'm not totally sure what is going on here. At that age he is as mature as he will ever be. If he does not have the personality and spirit to dominate them by now he never will. It sounds like a weak willed rooster with strong willed hens. If you can identify the strongest-willed dominant hen in this group you might try to remove her, at least temporarily, and see what happens. I'd suggest at least a week. It might not work, but then it might.
This is not a roster, he is still an immature cockerel. This sounds pretty normal. The dominant hen in this group wants to stay dominant so she doesn't allow him to mate with the pullets. At some point he should mature enough to take over.
You might read this link. It tells you how to look for the bull's eye when you crack an egg to tell if they are fertile. If the eggs you crack are fertile the ones you don't crack probably are too. Just because you don't see them mate doesn't mean they are not being sneaky.
http://www.backyardchickens.com/t/16008/how-to-tell-a-fertile-vs-infertile-egg-pictures