Answer: Master
He calls him 'sir' once or twice and even refers to himself as Frollo's 'son', but he calls him 'master' more than anything! It's really horrible how Frollo treats Quasimodo.
Frollo is a a powerful and ruthless judge who lusts after Esmeralda.
The relationship between Frollo and Quasi is very complex. Quasimodo looks to Frollo as a father, as a caretaker, when really Frollo is only using Quasimodo for his own benefit. He cares nothing about Quasimodo personally, he just feared his immortal soul would be damned for killing Quasimodo's mother, and is using this to counteract that sin. Quasimodo often defends Frollo, when Frollo only chooses to put Quasi down and torment him.
Frollo [referring to Quasimodo]: "Even this foul creature may yet prove one day to be of use to me."
Quasimodo [talking to the gargoyles]: "But you're all forgetting one big thing--my master: Frollo."
Frollo [to Quasi]: "This is my thanks for taking you in and raising you as my son?"
Quasi [in response]: "I'm sorry, sir..."