Introducing a new cock is difficult, they'll fight, or at least the dominant cock will assert himself on the younger one. The best success I had is introducing the pullets to the main flock at a couple of months old, they grow up together then and the cock doesn't seem to notice the other cocks. Until they start to tread, they might fight or the more dominant cock might just chase off the other bird when he tries to mate.
I'd just put them together and see what happens, I'd be surprised if the younger cock didn't back down quickly.