Jack is the apple of his father's eye. Wait. Do they have apples in the Amazon jungle? Perhaps we should say he's the piranha of his father's river? Or the anaconda of his father's tree?
No, that's wrong on so many levels.
Whatever the metaphor, Jack and his dad are super close. Even though Col. Fawcett is an absentee dad for most of Jack's young life, Jack never holds it against him. And when Daddy needs a man for one last Amazon expedition, he chooses his son. Jack is basically Col. Fawcett's mini-me: Grann describes him as a "tough, loyal" (1.16) young man who "yearned to be like his father" (12.7). In fact, Grann tells us, "Fawcett and Jack played their usual sports together, only now the son often surpassed the father in ability" (17.30).
Jack is strong, he is resistant to disease, and "Like his father, [he] was prone to contempt for others' frailty" (20.63). In fact, Jack sometimes takes his anger out on his best friend, Raleigh Rimmell, who doesn't have the same level of stamina as the Fawcett men. The Fawcett men aren't particularly nice men to know.
One way Jack and his dad are different, though, is in their ultimate motivations. Jack wants to find Z so that he will become famous, while Col. Fawcett wants to find Z for reasons unknown, though immortality seems to be a part of it. Well, just as Col. Fawcett is immortal in the sense that we still talk about him today, Jack, too, is famous for disappearing.
Be careful what you wish for.