It might help you to discuss what the actual purpose of using "sir" as a form of address is for. Is he trying to help you be more polite in society or the lifestyle? Is he trying to make you show him respect? Does he just want you to do it because everyone else is doing it? Is he into speech training/speech restriction?

If it's strictly out of respect for him, it may take awhile to feel like saying it, otherwise known as having earned your respect.