If we work backwards and examine the end message at the end of the novel, I think that it becomes evident how Bruno's development plays a role in this. Consider the closing words of the novel:
...of course all this happened a long time ago and nothing like that could ever happen again. Not in this day and age.
I think that Bruno's development as a character and his overall thematic significance is critical to this idea. Bruno is shown to be a character that emerges or grows into his notion of inclusion and respect. Boyne constructs his character to an extent that he ends up learning more about his world and the people in it. It is this process of learning that allows him to stand up for Shmuel and be an example of transcendent and powerfully compelling values in a world that might not respect such universality. Bruno is developed as a character that learns the value of respect, decency, and human dignity. In the absorption of these values, he ends up dying, but his sacrifice is one that enables the reader to grasp their importance. In constructing Bruno through such a manner, Boyne is able to educate the reader as to why it is important to hold such values even in a world that does not automatically show respect to them.