Is it necessary in this situation to use the future tense ("When will he be back?")?
A: Is Willy in?
B: He isn't. He is away in Lynn.
A: When is he back?
B: In a week.
A: I see.
If I were a native speaker of English, I would never shut up. :-)
Probably according to the grammar books, yes.
But "When is he back?" something you that you will HEAR.
So is "When is he going to be back?" or "When does he get back?"
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.