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.
In most English speakers' native grammar, "he" is a word that can't stand alone. That is why I think it sounds funny to some people's ears.
The reason for this, I think, is French influence on the language. In French, for a pronoun to be able to stand alone, it has to be in the stressed form, e g moi, vous, elle, eux. Our direct-object pronouns function in this same way.
I suspect quite strongly that they didn’t want it to necessarily make perfect sense (precedent: the Duolingo sentence “He ain’t heavy, he’s my brother”, where ‘ain’t’ shouldn’t be used). If I’m correct, it’s just meant to be a reference to the ‘Witch doctor’ song i.e. Who? He! Who ah ah, bing bang wolla e.t.c.
Australian native speaker again: Although " Wer? Er!" is the correct German pronoun, in English, although technically and grammatically, "he " is correct — in common English usage, native speakers would use " Him!" If you said; " Who? He!" Australian speakers would think you sounded idiotic, and certainly, they would think your English very unnatural.
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.
Shouldn't this be "Whom? He!"
That doesn't make sense in English -- "whom" is an object form but "he" is a subject form.
If you're asking after an object and answering, it would be "Whom? -- Him!"
If you're asking after a subject and answering, it would be "Who? -- He!" for traditional speakers, "Who? -- Him!" for many younger speakers.
The German can only be subject, since wer and er are both nominative, and German still strongly distinguishes between subject (nominative) and object (accusative) forms.
wen ("whom") is very much alive in German, even if it's obsolescent in English.