I don’t know how old your pullets are but some are oviously older than him. He is still pretty immature. It’s not at all unusual for older hens to pick on an immature cockerel.
First any more-mature chicken will outrank an immature chicken regardless of sex. That’s part of the pecking order thing. The older pullets dominate him because of that. But that’s only part of what is going on. With him being a young male and them females you have other stuff happening. Yes it is sex discrimination practiced by the females.
For a rooster to perform some of his flock master duties, he has to be the dominant chicken in the flock. For example, how can he keep peace in his flock and break up fights if the others turn around and beat him up? A rooster gets a lot of this respect by his glorious magnificence and tremendous self-confidence. He has to have an ego. The other part he gets by being the biggest and strongest. It’s something you can expect to see before too long. The mating ritual is about dominance as well as sexual. The one on top is dominating the one on the bottom. Some hens will squat for practically anything in spurs but others have to be forced to accept that dominance. I’ve had a 5 month old cockerel that had enough ego and magnificence to take over a flock. I’ve had one that took 11 months before he matured enough to do that. They are all different.
It’s not just his magnificence that wins over older hens either. A good flock master dances for his hens before he tries to mate them, finds them food and lets them eat first, provides security to the flock by watching for trouble (especially hawks), and keeps peace in his flock by breaking up fights. A young cockerel usually doesn’t do much of this, instead just being interested in mating for sex and dominance until he matures enough to get his hormones under control. Some of the older hens will probably withhold their favors until that teenager grows up and accepts his responsibilities. The more dominant hens may even beat the tar out of him when he oversteps his bounds, either getting fresh with them or just mating a willing hen. Consider it the older hens schooling a young cockerel on how he is supposed to behave.
With yours, it’s as if the hens can sense what is coming. They are going to get their licks in while they can. It’s probably in a big part schooling him since what you are seeing is pretty common in a flock with older hens and a young rooster. It doesn’t happen every time but it is pretty common.
You’ll probably start to see some changes in a month or two, maybe even sooner. He will start trying to mate the hens. Some may submit but many will resist. He will try to force some. In some cases he will succeed and in some cases he won’t. Eventually he will mature enough to take over the flock. When? Who knows? It depends on the personality of the cockerel and the hens. Sometimes it happens pretty quickly, sometime sit takes a year for him to win over the last hen. Expect it to get pretty hectic for a while then gradually calm down.
What do you do in the meantime? I see two options. You can keep him separate for a while, not so much for them to get used to him as to give him time to mature. Or, as long as the hens are not drawing blood, leave them alone. He’ll get picked on for a while but they will eventually work it out.
I don’t think he is a breed. I think he is a cross. I had one colored a lot like that a couple of years back. When his saddle and hackle feathers come in, I think you are really going to like the way he looks.
Good luck!