I think that Boyne's tone is revealed in the closing lines of the story. It is evident what Boyne's attitude has been throughout the construction of Bruno's narrative:
Of course all this happened a long time ago and nothing like that could ever happen again. Not in this day and age.
It is with this sentiment that Boyne reflects his tone throughout the story. His attitude towards Bruno is an embodiment of this idea. Bruno is constantly trying to make sense of his world, one that defies logic. He seeks to understand why things are the way they are. Those who offer faulty logic such as Gretel or Bruno's father end up suffering at the end. The author's attitude towards Bruno is one that shows the strength of human character. No one else in the story displays the courage and strength of character that Bruno does at the end when he holds Shmuel's hand and reaffirms their friendship in the gas chamber. Bruno shows no hesitation and little ambivalence in this moment. Boyne draws this stark contrast to what others have shown throughout the novel. Bruno's mother's ambivalence towards Auschwitz, the father's blind loyalty to it, and Gretel's changing perspective to be hope to be embraced by it are all reflections of the contingent and the temporary. It is here where the tone is most evident. Bruno speaks for universality and the absolute belief that the only resistance to injustice is the preaching of justice. This is in stark contrast to others who embody a sense of the contingent and temporary in the face of absolute terror. In Boyne's closing words, we see his affirmation of Bruno and in the fact that the ending of the novel spells personal disaster and emotional heartache for the family, it reflects his attitude towards both primary and secondary characters.