Update, I have culled 2 of the roosters (one infertile and this problem one) now I'm left with 10.5 month cockerel who is beautiful and docile, he will not mate a broody.Maybe, but maybe not.
If you put him in a pen alone but next to the flock, he can start to feel like part of the flock.
Or if you leave him with the other hens, and separate the broody, he will also start to feel like part of the flock.
If mating with the broody is only happening because he is new, then he should stop trying when he is used to being there and familiar with all the hens. If it continues even after he's settled in, you might consider whether he's really the right rooster for your flock or whether you would be better off with a different rooster.