Depends on context.
When replying to 'Whom did you see?' you'd say 'Who? Him!', but when replying to 'Who did it?' you'd say (somewhat awkwardly) 'Who? He!'
Whether it's 'him' or 'he' depends on whether the person you're talking about was the subject or the object of the previous sentence.
Not when I am speaking, but then I also say "It is me" rather than "It is I"
Good try but no one says "Who? He!" they might say he did, it's him, he can etc
"Who he?" is often used in (UK) English as a shorthand for Who is that? when you see someone you don't recognise.
I feel like there might be a mnemonic devise here. You can think of the nominative "wer" as being similar to the nominative "er," just with a w added to make all question-sounding. Similarly, "when" sounds a little like "ihn," if you use your imagination. That's how I remember it at least.
Why (wen) is wrong ?? Can someone please explain the differences between Wann/wenn Wen/wer
wen is the accusative case. You would use it when you are asking for the direct object -- the answer will be something like "me" or "him". For example, Wen hast du gesehen? "Whom did you see?"
But since the answer here is "he", you're asking for the subject, as in "Who saw the man?" *Wer hat den Mann gesehen?"
Are you Greek? Then wen/wer is like ποιον/ποιος: Ποιον είδες; Ποιος είδε τον άντρα;