That's normal, especially if they weren't raised with roosters, but even then the alpha hens have to remind the cockerel who is who. If he's under a year old hes still a cockerel. Roosters do not reach their adult size before a year old, some take as long as two years, same with hens, though they reach adult or close to adult height quickly. He will get bigger and heavier soon and once he's not so young he'll be respected.
Even if he grows up to dominate other roosters he will not be alpha over the hens. That's a common misperception people have. There will always be an alpha hen, and she will always dominate juveniles, and the hens just under her on the hierarchy will also dominate juveniles. In fact any adult chook will dominate the juveniles. Once he's a grown up rooster, usually around a year old, he will be accepted. Until then he's just an upstart teenager. If they keep attacking him beyond that age, there is either something wrong with him or them.
