However, Son decided to enter the Unicorns after graduation from college and continued to play baseball as a pitcher at Yeungnam University in Daegu.
As a sophomore in 2002, Son was called up to the Under-22 South Korea national team for the inaugural World University Baseball Championship held in Messina, Italy.
In 2003, Son was selected for the South Korean national team as an amateur player and participated in the Baseball World Cup held in Havana, Cuba.
At the World Cup, he led team in innings pitched with 18.1 and earned a save against Italy in the round-robin stage.
In the Nexen Heroes, which bought the right of the Hyundai Unicorns in 2008, Son made a transition to the closing job and posted a record of 2–3 with 26 saves and a 2.56 ERA.