Ah, the patriarch. Even though he begins the novel attempting to be strong and tough, for most of the novel Wang Lung's father is like a child. He's sick from the very first line of the book: “The house was still except for the faint, gasping cough of his old father, whose room was opposite to his own across the middle room. Every morning the old man's cough was the first sound to be heard" (1.1). By the end, though, all he can think about is food and drink, even more than about the birth of his grandchildren or even, at first, the death of O-lan.
We get a few glimpses into Grandpa's past, but not very many. We know that his wife died six years before the beginning of the novel, and apparently their lives were very hard. Wang Lung is his only son, and he tells him, "Ah me, to think that out of all the children I begot and your mother bore, one after the other—a score or so—I forget—only you have lived!" (3.26).
Through Grandpa, we can also see where Wang Lung gets his pride. When the family goes to the South, he chooses not to beg, saying basically that he's already worked enough in his life: and only says, "‘I have plowed and I have sown seed and I have reaped harvest and thus have I filled my rice bowl. And I have beyond this begotten a son and son's sons.' And with this he trusted like a child that now he would be fed, seeing that he had a son and grandsons" (11.66).
Even though Wang Lung changes as he gets richer, his father remains traditional until the end. Wang Lung sees nothing wrong with taking a second wife, but his dad totally does. He screams at Lotus and spits at her, calling her a harlot. For all his rigidity, it's also possibly true that Grandpa is one who best understands O-lan's worth. Maybe that's why he is so sad when O-lan dies. He dies shortly after she does; when they go, it's as if the traditional part of Wang Lung's life is over.