Despite never earning a provincial call-up at high school level, Van der Merwe joined the Blue Bulls Academy after finishing high school and represented the Blue Bulls U19 side in the 2014 Under-19 Provincial Championship, starting eight matches and making three substitute appearances as they reached the final of the competition, where they were defeated 26–33 by Western Province U19.[2]
He made his first class debut for the Blue Bulls on 6 March 2015, coming on as a late replacement in their 2015 Vodacom Cup match against the Falcons in Kempton Park, helping them to a 37–13 victory.[3]
He also played for UP Tuks in the 2015 Varsity Cup competition, starting their match against UJ[4] and playing off the bench on two occasions.
Despite initially not being named in a 37-man training squad for the South Africa national under-20 rugby union team,[5] he did feature for them in a friendly match against a Varsity Cup Dream Team in April 2015. He came on as a first-half replacement and scored a try just after half-time in a 31–24 victory.[6] He was also included in the squad that embarked on a two-match tour of Argentina.[7] He came on as a replacement in their 25–22 victory over Argentina[8] and started their second tour match a few days later, in which he scored two first-half tries in a 39–28 victory.[9]
Upon the team's return, he was named in the final squad for the 2015 World Rugby Under 20 Championship.[10] He started their opening match in Pool B of the competition and got South Africa's first points in the competition by scoring a try in a 33–5 win against hosts Italy.[11] He played off the bench in their remaining two group stage fixtures, a 40–8 win against Samoa[12] and a 46–13 win over Australia[13] to help South Africa finish top of Pool B to qualify for the semi-finals with the best record pool stage of all the teams in the competition. Van der Merwe came on as a replacement in their semi-final match against England, but could not prevent them losing 20–28 to be eliminated from the competition by England for the second year in succession.[14] He started their third-place play-off match against France, helping South Africa to a 31–18 win to secure third place in the competition.[15]