Indeed, he was so shocked by her accusation that he was content to fall backwards, to get as far away from her as he could possibly get, without fleeing the flat altogether! He owed her at least the news of their mother's death, and of their father's ostensible 'illness'. He also owed her a great deal more than that-- such as a safe haven from this life of whoredom, and more importantly, a safe haven from Papa!
"Di... please... don't say that! I'm not Papa, and I don't intend to hurt you like he did!" He struggled to get up and away from this strange young man who he had just barrelled into, with a murmured apology.