Bush was born in 1985 in Spring Valley in San Diego County, California, and named after his biological father, Reginald Alfred Bush, Sr. He was a running back at Helix High School in La Mesa, California. While at Helix, he played with 2004 Heisman Trophy finalist and first overall pick in the 2005 NFL Draft Alex Smith. During his prep career with the Helix Highlanders, he won the prestigious Silver Pigskin trophy awarded by KUSI's Prep Pigskin Report. Smith and Bush were later finalists for the 2004 Heisman, making it the first time a high school had two finalists at the same ceremony. He played in the 2003 U.S. Army All-American Bowl.