The author has already revealed that Herbert has been mangled in the machinery at work, so we can assume that his body is no longer the way the Whites remember him. When Mr. White made the second wish--"I wish my son alive again"--he did not fully comprehend the possibilities, just as he had not considered the awful implications that could come with the first wish. He had been badgered by his wife to make the wish out of their own sadness and desperation. Only after he makes the second wish and hears the knock on the door does Mr. White realize that the knock comes from Herbert--and that he will be in the same state in which he died, not as the healthy son they remember. Mr. White understands that if he opens the door, it will be the returned-from-the-dead Herbert, complete with the terrible injuries he received in the accident; so, he makes the third wish--presumably that Herbert be dead once again--in order to spare his wife from any further horrors.