When the father shared a Coca Cola with his son, what is the significance of it?

Well, these two are near starving for much of the book. Until they find the underground bunker, they keep each other going through this intense love they have for each other. One of my favourite parts is when the father finds a Coke in a machine. The boy drinks it but insists his father drink as well. It's a tender tribute of their concern for one another that transcends thirst, hunger, or even survival.