"Who? Him!" should work in English. It didn't for me, anyway.
It's not part of the native speech of many native speakers, but it is part of the standard formal language.
There are many "Englishes", each with their own grammar, which is mostly - but not completely - the same.
I can't come with a grammatical rule that would make it incorrect, but definitely akward enough I think maybe someone with a better grasp of grammar could explain why.
Strictly speaking, the whole phrase is incorrect, because it's a pair of sentence fragments. Ignoring that, no actual rule dictating which is right (if anything, he would be correct because whom wasn't used), but it sounds awkward and few natives actually care or know about who vs whom.
I think it's because we would say, "Who did it? He did it? It's leaving he in the nominitive case because you're asking about a subject.
Because "Whom?" is used to ask after an object -- equivalent to Wen? in German for the direct object.
But Duo's sentence has Wer?, which is in the nominative case and used to ask after a subject, so the English equivalent is "Who?".
Preferring the translation "Who? He!" is teaching people bad English grammar while they learn German. Cite all the rules you like, but idiomatic and spoken language matters, and "Who? He!" would sound bizarre at worst, stilted and foreign at best.
Wer? Er! Wie? Sie! Wes? Es? (Also "wes" was not marked wrong, is that something I shouldn't say?)
I assume you're joking; wie means "who" in Dutch but not in German, where it means "how".
(Also "wes" was not marked wrong, is that something I shouldn't say?)
It's an old form of the word wessen "whose", which nowadays is only found in fixed expressions or proverbs, such as Wes Brot ich ess, des Lied ich sing (= Wessen Brot ich esse, dessen Lied singe ich): literally, something like "Whose bread I eat, his song I sing", similar to "Who pays the piper calls the tune": indicating that people will tend to obey people who give them things.
I'm pretty sure, who? He? Is correct English gammer. If you and more to the sentence it becomes clear. Q. Who is the culprit? A. He is the culprit. You would not say, Him is the culprit. So he is correct, but not often used so sounds weird.