Teacher Eco's answer is generally true for English language teachers. But there are exceptions to this. A teacher in elementary school or even high school will probably want the students to call him Mr. / Ms. / Mrs. X. And I think some university professors are very proud and want to be called Professor X, even the young English ones. You can ask him and find out for sure. Just say, "Excuse me, what should I call you? Professor X? Or...?"