Upon the team's return, he was named in the final squad for the 2015 World Rugby Under 20 Championship. He started all three of their matches in Pool B of the competition and was also the main kicker for South Africa throughout the competition. He kicked three conversions in a 33–5 win against hosts Italy in the opening match, three penalties and two conversions in a 40–8 win against Samoa and two penalties and four conversions in their final match against Australia, also scoring a try in the 55th minute of their 46–13 win. The results meant that South Africa finished top of Pool B to qualify for the semi-finals with the best record in the pool stage of all the teams in the competition. Thomson also started their semi-final match against England, kicking two conversions and two penalties, but couldn't prevent them losing the match 20–28 to be eliminated from the competition by England for the second year in succession. He started their third-place play-off match against France, with his four conversions and one penalty helping South Africa to a 31–18 win to win the bronze medal. He ended the competition as the top scorer in the tournament, with 59 points.