Any terser expression for this?

"The boy who survived the great catastrophe" is one possibility.

You can't use "survived" as an adjective like that. The boy survivor maybe.

You could say "the surviving boy".

Yes, I know, but the difficulty there is that this implies that he was either the only boy who survived out of all the people or the only survivor out of a group of boys. In fact, going back to the original sentence, the use of the definite article "the" implies some kind of uniqueness about the boy - that he was either the only surviving boy or the only survivor. Unless this is the intention, it would be better to use the indefinite article "A".

Bada bing.

Just FYI Oristar, terse, while technically correct in the context you use it, has, in English, I think, an implication of shortness of temper or possibly rudeness that means I'd tend to use "more concise" or "less wordy", for instance in its place.

Is this meant to imply agreement?

Of course. Oristar used the definite article so I assume he meant to.

A shaky assumption.

Not the shaky assumption?

Thank you all.

In my opinion, possibly shaky.

So nothing unusual then.