Dommy ... Mommy? Not quite but close. The fact that she was raping him from young (maybe the dad put her up to this? I wouldn't doubt it) and that she immediately took him to her room, stripped him, and tied him up (he didn't fight back? Okay) means she missed it? Plus that math didn't add up. The equation of his words and penis was unsolved. Talking a lot of smack and being ROCK hard does not compute.
And yes, before anyone goes with an "um actually" I AM aware that the human body sometimes doesn't do what your conscious mind 'wants' when being sexually assaulted. He couldn't help it since he was already groomed.