"Is he waiting for someone?"
I marked the feminine, too. I think it's because if one is speaking in general, the pronoun defaults to the masculine.
In Italian "per" isn't needed to complete the meaning of "aspettare", so it would introduce a different object, i.e. it'd sound like he's waiting, and he's doing it for (or in place of) someone.
The english would imply the phrase Sta aspettando qualcuno. Aspetta qualcuna in english would be he waits for someone
I'm a native Spanish speaker & in this sentence "por" (Spanish for "per) would be needed otherwise it changes the meaning to "someone is waiting" (está esperando alguien). How do you know in Italian that the one waiting isn't "qualcuno"?