"The man had been outside" - subtly different, but still correct? (it's not accepted).
You are right, Peter! This is subtly different. The difference is in the tenses of the sentences. The difference between "has" and "had" is a progressive step further back in time. So, what has just happened is not the same as what had happened a little while ago.
To get a bit nerdy, the sentence "l'uomo è stato fuori" is in the "Passato prossimo" Italian tense. (This equates to the English "Present perfect") Your reply "The man had been outside" is further in the past and is in the English "Past perfect" tense (also known as the pluperfect indicative just to test your patience even more!) The past perfect tense in Italian is the "Trapassato prossimo"
I just think of this lot as "The HAD tense" to simplify things. To say your answer "The man had been outside" would be "L'uomo era stato fuori." As you say, subtly different!