First of all, you are attempting something that is pretty difficult to do, maybe impossible. Introducing a rooster to a flock led by an established rooster is dicey at best, definitely not the same as introducing a new hen to a flock. My two main roosters won't even share and get along and they were together in the same flock until the son grew up as large as his father, but now, they'd fight to the death over their respective hens. He went into the flock as a young cockerel, with me hoping that his father would allow him to stay, but it just did not work out. I've tried this before with other roosters, sneaking in a young son and hoping that would work out, but no luck so far, had to mop up blood and give up eventually. Bringing in a completely strange rooster to one who is possessive of his hens and may treat the new one as any other predator may be a sticky situation.
Even doing as the previous poster suggested may not work. Some roosters will just never allow some interloper in with their hens. If you decide to do this anyway, quarantine this new one for at least 4-6 weeks AWAY from your flock, not close by. Best of luck to you!