When Will lies to the woman on the plane about his father's identity, he's exposing a difficulty that his lack of paternal support has created for him. Because he doesn't know almost anything about his father, he's created a variety of stories about the man in his head. He could be the president of a company, someone he passes on the street, or anyone else, really.

Not knowing his father has left such a large gap in Will's life that in many ways, he doesn't feel that he knows himself. He doesn't know what parts of him are from his mother's heritage and what parts of him are from his father's. He'll never be able to find an answer for that.

Even for people who know their parents and have close relationships with their families, the question of who you are as an individual and which aspects of your personality come from your heritage and upbringing often arise. In this way, it's easy for a reader to relate with Will's struggle to come to terms with the disparate parts of himself. Anyone who's ever wondered who they were and what it means to be themselves can probably find aspects of that in Will's actions—though his decisions are often more extreme.