Sorry... immature pullets yes. Even their own brood mates will try to breed them at that age as the boys often mature sexually faster than the girls do.

Young cockerels are the worst.
But so far, I haven't had them (rooster/cockerel), go after
chicks that a broody was raising with the intent to breed them or harm them. Of course that could change tomorrow. Each day is a new experience... And each chicken is an individual.
12 weeks means the girls are getting close to breeding age even if they aren't ready yet. The boys can sense these hormonal changes that we cannot. But you may notice the combs starting to turn pinker. The boys will be attracted to and want to go after the ones they can tell are highly fertile... or at least almost.
I might not take his head off just yet! Although I would expect him to continue trying to mate them, unless separated until after they are ready at which point he will still try to mate them.

Eventually they will submit to him and he will do his thing, if that's what you allow. Although I think it looks/sounds brutal (and I personally won't allow it towards girls of yours age), I will say that roosters don't have penises so it is kinda like rubbing belly buttons. But that doesn't mean that injury cannot occur.
If he is OK with your other ladies (not too rough, doesn't focus on a favorite girl too much, calls them to treats, stands guard while they dust bathe, drops his wing and does a little shuffle and serenade) and you like having him around.. I would find a way to keep the young & him apart or let em work it out. Or like you say try again in a couple of weeks. He will still mature a bit more from this point. So if he isn't human aggressive by now, that's actually a really good starting point!
Prior to him harassing the young pullets, they weren't putting out certain hormonal vibes... would be my true assessment. Thank you very much for sharing your experience! I hope this helps you to feel better about your boy and/or make a decision either way that's in the best interest of your flock.
Keeping roosters isn't for everybody. I keep my roosters separate and let them mingle only when I want them to, partly because they will also often target the weaker/more submissive girls. But also because I wish to control my breeding genetics. But several times the thought of not dealing with roos has crossed my mind because my girls are plenty happy, if not even more at peace without some horny ol' roo (doing what comes natural) pestering them.

Some of the girls do enjoy hanging around the boys to though.