The fat man focuses on the wishes of his son throughout this whole argument. Why does the lady's question about his sons death destroy his optimistic outlook, emotional stability, and composure?

A group of travelers are on board a train heading for Rome. All of them have been affected in one way or another by the ongoing war. A lady has just come aboard with her husband; they are traveling to Rome to see off their only son, a 20-year-old student who's unexpectedly been conscripted. A number of other passengers share similar experiences, and an unseemly squabble ensues, with parents engaging in a Dutch auction as to which one has suffered the most.
Then a fat man with large, bloodshot eyes, chips in. He loudly proclaims that children do not belong to their parents, but to their country. It's perfectly natural, he goes on, that young men should love other things besides their parents, and that includes their country. Almost in passing, the fat man reveals that his son was killed at the front. Though he doesn't mourn for him because he died happy, without having seen the ugly side of life.
After a brief period of silence, the woman who boarded the train with her husband asks the fat man a question:

Then . . . is your son really dead?

At this, the fat man suddenly breaks down, and bursts into uncontrollable sobs. He seems to have realized that he really didn't know what his son wanted after all. All this time, he's simply projected his own feelings onto his dead son as a way of suppressing the grief which has now finally come to the surface. Before he died, the fat man's son sent him a message saying that he was "dying satisfied at having ended his life in the best way he could have wished." But his son was only trying to reassure him; he wasn't expressing some kind of death wish. The woman's simple question forces the fat man to realize this, and it destroys him inside.