"Nothing." -Xavier, answering the question "What are you doing?" [src]
Xavier is one of Candace Flynn's future children, who appears in "Phineas and Ferb's Quantum Boogaloo". He is the son with bright blue eyes and blond hair, which gives him an appearance similar to his presumable father, Jeremy Johnson. He and his brother Fred are generally rather inactive, and constantly like to make note on how they're just sitting in front of the digital tree in their family's backyard, much to the annoyance of their sister Amanda. Their life takes a turning point, though, when they by chance get a visit from the younger-aged versions of their uncles, Phineas and Ferb. After the aforementioned visit, Xavier and Fred construct bumper cars that can move in five dimensions, but they still answer Amanda's query about their activities with "Nothing."
Xavier and Fred are as bright and creative, and build impossible stuff just like Phineas and Ferb, although it took the younger version of their uncles to bring it out of them.
"Amanda" and "Xavier" are the two names that Candace had picked out for her future children. Given her interest in Jeremy, it's probable that he is the father of both children. However, this is not definitive.
If Jeremy Johnson is Xavier's father, that would mean Suzy Johnson would be his aunt. He would be more distantly related to Nicolette too (Jeremy's cousin) ("De Plane! De Plane!").