Once an Alpha claims a boy for his own, a natural synchronizing transformation begins taking place. By diligently keeping him in full chastity, the boy shifts the attention he used to give his own dick onto his owner's cock. He becomes attuned to its needs, and fervent about stimulating it and making it cum (much the same as he used to, though mistakenly, for his own!).
As his pussy gets abused more and more, his brain understands this is where pleasure comes from. Perceiving the sex drive of his owner triggers a response which readies the boycunt for insertion, automatically relaxing the sphincters, and swelling the anal region for increased stimulation. The swelling causes an odd discomfort in the boy which draws his attention and prompts him to pursue relief. This is called the "boy heat", as the boy becomes desperate to get his swollen lips used.