回答
Both are correct. It just depends on the situation. When you are asking about a man you do not know, you say, “Who is he?” In this case, "he" is the pronoun referring to the man you do not know. You might have just said "Hi" to him in an elevator, but you did not have a proper introduction, and you still do not know him.
When you want to ask which man is the one you have heard of, you say, “Who is him?” In this case, "him" is the pronoun referring to the man you have heard of. For example, your company hires 5 new associates. Mary tells you that one of them is going to join her department. You and Mary go to a meeting where all the new 5 associates will be introduced. You ask Mary, “Who is him?” Alternatively you can ask, “Which one is him?”
In Japanese, we say, 彼は誰 and 誰が彼, respectively.