What might the man mean when he says to himself, “can you do it?when the time comes?can you?”

The man must reconcile with the fact that he will have to shoot his own son when the time comes. The world is disintegrating before their eyes. There is no humanity left: the boy is perhaps the last vestiges of it. There will come a time when "bad people" will find them and the man will have to shoot his own son to spare his suffering. The man wonders if he, in the end, will have the courage to do it..