The fat man operates as the force that tells parents the nobility of a child dying in name of country. When he hears of the parent son the train feeling despondent over sending their children to fight in the war effort, he enters the discourse with a sense of totality and absolutism in his speech. Simply put, he purports to have "the answer." This certainty is powerful for in the discussion of doubt and insecurity, he is able to assert a sense of unquestioning authority. He details his own son's experience at being proud of giving his life for country. The feeling of national identity and patriotism begins to infect the atmosphere of the railway car. At a certain point in the dialogue, the reader actually feels that some order has been restored to a setting where disorder had been running amok. It is only when the woman asks if his son is "really dead," that disorder and disunity emerges with a brutal supremacy as the father realizes, through a rather twisted notion of epiphany, that he has lost his son, never to return. The finality of death is war becomes the only certainty in this particular setting.
I do not think we know what his convicitions are. We hear what he says they are, but then we realize he's not really telling the truth about his feelings.
He says he does not even mourn for his son because his son died for his country. This is a noble death, he seems to be saying and so there is no need to mourn. If this is how he really feels, it is a very patriotic sentiment. So you would say he is a true patriot.
But then he breaks down when the woman asks if his son is really dead. This tells me he didn't really believe that stuff he was saying.
So I don't know what convictions he has -- I just know he's a parent who is devastated by the loss of his son.