Yes, he is. The U.S. Constitution forbids U.S. citizens from receiving honors such as this, but, of course, does nothing to prohibit non-citizens from the same. It is a matter of courtesy to use someone's title, even in the United States. Even the U.S. government refers to people with their titles, such as The Right Honourable Stephen Harper when the Prime Minister of Canada visits the United States.
"Sir," especially in an instance such as this, is an earned title with which its owner is entitled to be addressed.
Actually the Constitution doesn't forbid US Citizens from receiving such honors. You might be thinking of the Titles of Nobility Amendment, which never was ratified by the required number of States.
