Scifiwire posted that Patrick Stewart will be knighted by Elizabeth II, Queen of England, as the actor is on the New Year's Honour's list according to anonymous sources. The official list has not been announced at this time.
Stewart's career boasts over 100 appearances in film and television, and is best known as Captain Jean-Luc Picard on Star Trek: The Next Generation, and as Charles Xavier in the X-Men franchise. That doesn't include his impressive track record on the stage; Stewart became a member of the Royal Shakespeare Company in 1966, the same year he first appeared on television.
According to the report, the Queen is a fan of the 69 year old actor. I wonder if it's for his Claudius (Hamlet) or his Jean-Luc.
So whether you mistake him for Captain Picard, or his extensive work as an actor, calling him "sir" is quite appropriate.