War doesn't work as a comparison, it's a small child. A missing child is going to die, and there is no hope. Closure is the best you can have there. Tell her it was peaceful and that he tried to save his dad. Lie a little
As I stated in different wordings, having a closure is nice, but how people react to that closure can be what you do not call "nice".
Sure the kid is likely just turns out to be dead if he's missing. But without knowing anything, people around the mother can talk her into a state where she did not give in to despair, compared to knowing the kid is already dead. The kid may have run away and gotten picked up by a traveling troupe of circus, the kid might have just found himself a way to hunt/trap/gather for his own needs in the wild, there can be a lot of ways to talk it out. Of course, that's when the information that he is already dead not available.
Why do I emphasize on that? Because closure is not the end. And having closure for the mother might mean you have a dead woman on your hands, or worse.
It's not about closure. It's about what comes after that closure.
That's why, sometimes, inaction might not be that bad.