Fred is one of Candace Flynn's future children, who appears in "Phineas and Ferb's Quantum Boogaloo". He is the son with dark blue eyes and brown hair. He and his brother Xavier 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. Fred keeps talking about being next to something (Fred: "I'm sitting next to him","I'm sitting next to it","I like the guy who's standing next to him"). 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."
In a recurring joke from previous episodes, Candace mentions how in the future she will marry Jeremy and have two kids, Xavier and Amanda. Fred's inclusion is almost certainly meant to be a play on this, alternatively implying that Candace eventually changed her plans or simply had more children than she had expected, or because she didn't get a girl. Another theory is that Fred is Candace's stepson, which would also serve to further the similarities between Candace's future family and the present-day Flynn-Fletchers. He and Xavier appear about the same age, implying they may be twins if not stepbrothers.
Fred and Xavier 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.