Bostock chose not to sign, stayed in college, and began playing baseball for Coach Hiegert and the Matadors. He was an all-conference player in the California Collegiate Athletic Association in both of his seasons at San Fernando Valley, hitting .344 as a junior and .296 as a senior, leading the Matadors to a second-place finish at the Division II College World Series in 1972. He was selected by the Twins in the 26th round (596th overall) of the 1972 amateur draft and decided to turn professional, just 15 credits short of finishing his bachelor's degree.