"Ow! What was that, Dad? It felt like something tore."
"Don't worry, son, you'll never have to worry about that again."
Dad knew he'd shredded his son's hymen. He'd explain it all to him after he made sure he was knocked up for sure.
He knew knocking up his son would mean the boy would never experience a true ejaculation & caging his son's dinky would help that process along, since his son's body would under go permanent changes to accept the cum of MEN.
That is exactly what he wanted. The boy shouldn't be burdened with the memory of what he lost. May he imagine what the explosive orgasms of his father felt like? Assuredly he would. He may even think that his girly contracting pussygasms on his father's dick could be similar. He would be wrong of course, his feminine pussygasms would be but rolling hills to the towering mountain of a man's ejaculation.
That is what his father wanted. To keep his boy pure from the burdens of manhood.