I am an Anglophile and have been all my life. Still, is it necessary to call a Brit or anyone by a title? Americans do not have nobility or royalty, de jure. Call him, or any other ennobled Brit, by his last name, or first, if you're familiar. As I said, I'm an Anglophile but we don't use fancy titles for Rothchilds and anyone in the Almanach de Gotha.
No reason for an American to fawn over someone, even a cousin.