He is not a mature rooster, he is an immature cockerel. The hen knocking him off is dominant. The mating act is as much if not more about dominance as it is about sex. The one on bottom is accepting the dominance of the one on top, either willingly or by force. Even if your EE will accept that cockerel as dominant your head hen or other mature hens may not. She keeps him in his subordinate role by knocking him off whenever he tries to mate.
At some point he will mature enough to take over as flock master. I've had one do that at 5 months, another waited until 11 months so he is about due. The personality of the dominant hen has something to do with that timing too. Some hens can be pretty domineering. When he matures enough to stand up to her his takeover of the dominant position may go very smoothly or it could get violent.
Since I play with genetics I typically keep a new cockerel every year so I've been through this a lot. Most of the time it goes so smooth I hardly notice but with that 11 month old he and the hen fought off and on for two days before she finally accepted him as dominant. They then become best buddies.