In 2013, Coetzee was selected in the South Africa Under-20 team that attempted to retain the IRB Junior World Championship title that they won in 2012. He played off the bench in their opening match of the competition as they beat the United States 97–0 and in their next match against England in their 31–24 victory. He started their final pool match against hosts France, helping his side to a 26–19 victory which ensured South Africa finished top of their pool and qualified to the semi-finals. He was an unused replacement as South Africa lost their semi-final match 17–18 to Wales, but once again played off the bench as South Africa clinched third spot in the competition by beating New Zealand 41–34, to win their fourth third-place play-off match in six years.