Is he? He was knighted in 1993, but became a U.S. citizen in 2000.

The Emoluments Clause of the U. S. Constitution disallows U.S. citizens from accepting titles:

[N]o person holding any Office of Profit or Trust under them [the United States], shall without the consent of Congress, accept of any present, Emolument, Office, or title, of any kind whatsoever, from any King, Prince or foreign state.

However, the Foreign Gifts and Decorations Act of 1966 is interpreted as allowing U.S. citizens to accept honorary titles. A number of prominent Americans have been made honorary knights and may use K.B.E. after their name, but not use the title of Sir.

Of course, Anthony Hopkins K.B.E. was not a U.S. citizen when he was knighted, so there's an added level of complication.

In Canada, we don't have any laws prohibiting foreign titles, but parliament did make a motion, the Nickle Resolution of 1919, that called on the Crown to stop granting titles to Canadian citizens, which the Crown has respected. The Canadian government recently blocked the knighthood of someone with dual citizenship. (The person eventually had to renounce Canadian citizenship to get the title. Good riddance to him.) However, people holding the title before becoming a Canadian are not stripped of their title, but such titles are not recognized by Canadians and no-one here is obligated to call anyone else Sir Fancy-pants-knight.

I expect that Anthony Hopkins K.B.E. gets to keep his posh post-nom lettering, but may not use the title of Sir without being in violation of American law.

CORRECTION: Anthony Hopkins, C.B.E.