The message that the old man's son sends to him justifies his participation in the war. While his father believes the son's words, it does not ease the pain over his death.
The old man carries himself with a great deal of confidence as he speaks to his fellow train passengers. He hears the sad discussion from the passengers about the sacrifices their children are making. The old man affirms that "everyone should stop crying" because these are "good boys" answering the call of their country. The old man references his own son as evidence. He tells his fellow passengers that his son "before dying, sent me a message that he was dying satisfied at having ended his life in the best way he could have wished." As further proof of this, he shows the passengers that he does not "even wear mourning." The old man solidly believes that his son died for a proper cause. The old man believes this sacrifice is not a source of sadness as much as it is a source of pride.
While the old man believes the words his son wrote, it is clear that he has not fully processed the boy's death. When the woman asks him if his son is "really dead," it triggers powerful feelings. His "harrowing, heart-breaking, uncontrollable sobs" show that while the old man might believe the words his son wrote, they do not provide consolation. He has lost his boy. The war took his child. No amount of justification can lessen that hurt. As a result, the ending to Pirandello's story speaks to the unending pain that is a part of the war experience. While claims of nationalism and patriotic duty can be used to justify it, nothing can mask the pain that war brings on those who have to experience it, something the old man demonstrates.