"How is this man doing?"
Translation:Wie geht es diesem Mann?
Yes, es is the impersonal subject (in nominative), and the real subject is in dative here (as we ask for the condition / state to which it is subjected, and German uses for this in a few instances the dative).
Wie geht es ihm? – Ihm geht es gut. / Ihm ist bitterkalt. / etc.
but: Er ist krank. / Er ist in Ordnung. / etc.
I still use the apostrophe to abbreviate es, but apparently the spelling reform allows to omit it just as the contraction of prepositions and definite articles (e.g. in das = ins) does.
Is that an actual question or was that your answer and you're asking why it's incorrect?