He is afraid of his own son. He saw the mangled remains of his dead son and knows that he can only be a horrible monster if brought back to life. The reader can only imagine what the son looks like--if that is the son knocking at the door!! And who else would it be? Who else would be knocking at the door in the middle of the night after Mr. White, at his wife's insistence, had wished for Herbert to be alive again? What the reader is imagining is not only a Herbert who is mangled and shredded, but a Herbert who is not really "alive" but a sort of zombie who will never possess the same bouyant personality after suffering incredible pain and actually being dead. The reader imagines Herbert's appearance and demeanour through the mind of his father and can understand why Mr. White countermands his second wish as he hears his wife drawing the creaking bolt slowly back.