The boy's father just doesn't understand that his young son has made such a mistake. It would probably never occur to a small child that there are two different kinds of temperature measurements, Celsius and Fahrenheit. And it would be highly unlikely to occur to a parent that the child had made such a mistake. The father doesn't have the slightest idea what is going on in his son's mind until near the end, when the boy asks:
"About what time do you think I'm going to die?"
When the amazed father tries to assure his son that he isn't going to die, the boy replies:
"I know they do. At school in France the boys told me you can't live with forty-four degrees. I've got a hundred and two."
The father realizes that his son had been waiting to die all day. Which explains the title, "A Day's Wait." We admire the little boy's fortitude. We also remember how many such mistakes we made in our own lives, and are reminded what a tough job it is just growing up in this complex world.
It is rather touching to see this soft side of tough Ernest Hemingway. But his work as a whole is noted for its combination of toughness and sensitivity.