How does Paul greet Keller? How is this different? What shift has occurred in their relationship?

Paul greets Keller in German.... his words are respectful. In the end, Paul reats Keller as a son would treat his father, rather than the way a student treats his teacher.

‘Wie gehts, Maestro,’ I whispered, the word for the first time sounding upper-case in my mouth, respectful.