Van der Merwe was born and grew up in George in the Western Cape province of South Africa. He attended and played rugby for Hoërskool Outeniqua, earning several provincial colours by representing the at youth tournaments. In 2011, he represented SWD at the Under-16 Grant Khomo Week held in Queenstown, appearing in all three matches and scoring a try in their 25–8 victory over KwaZulu-Natal. The following year, he played for SWD at the premier schoolboy rugby tournament in South Africa, the Under-18 Craven Week. He scored four tries – two against the and two against – at the tournament held in Port Elizabeth. He was named in the South Africa Schools team at the conclusion of the tournament, and made two starts for them in the Under-18 International Series, helping them to victories over France and England. Still eligible for the Under-18s in 2013, Van der Merwe again played in all three of SWD's matches at the Craven Week held in Polokwane. He scored three tries – two against Eastern Province and one in a victory over the – and was again included in the South African Schools team. He scored a try in their 19–14 victory over England in their first match of the 2013 Under-18 International Series, and followed that up with two tries in their next match against France. He didn't score in their final match against Wales, but still finished as the top try scorer in the competition. He also played rugby sevens, where he represented South Africa.
2014–2016 : Youth rugby
After high school, Van der Merwe moved to Pretoria, where he joined the ' academy for the 2014 season. Shortly after joining, he was a late call-up to the South Africa Under-20 squad that participated at the 2014 IRB Junior World Championship in New Zealand, replacing Rohan Janse van Rensburg who picked up an injury in South Africa's 61–5 victory against Scotland in the opening round in Pool C. Van der Merwe was an unused replacement for their 33–24 victory over hosts New Zealand in their second match, but started their final pool match, a 21–8 victory over Samoa to help the team finish top of their pool to progress to the semi-finals. He didn't feature in their 32–25 win over New Zealand at that stage, but was named on the bench for the final against England and came on as a replacement in the second half, with South Africa losing 20–21 to finish as runners-up in the competition. He returned to domestic action to play for the team in the 2014 Under-19 Provincial Championship. He made twelve starts in the competition and scored a total of eight tries, the second-highest by a Blue Bulls player and joint-third overall in Group A of the competition. After scoring a single try in matches against and, he scored two tries against the in a 52–27 win. Another try in their second meeting against Western Province was followed by a hat-trick in a 46–24 victory over the Leopards in Potchefstroom. The helped his side finish top of the log and make it all the way to the final of the competition before losing to Western Province. In addition to his appearance for the Under-19 team, he also played one match for the side, in a 24–10 victory over. He had an operation at the start of 2015, which ruled him out of action for the majority of the season, also ruling him out of contention with a second consecutive Under 20 Championship. He returned to action for the s, making six starts in the Under-21 Provincial Championship, scoring one try in their 43–10 victory over. At the start of 2016, he played Varsity Cup rugby with, starting all eight of their matches. He scored tries in defeats to and and scored two tries in their 68–25 victory over. In their final match of the regular season against bottom team, Van der Merwe scored six of his side's eleven tries in a 100–25 victory. With bonus points in effect for scoring tries that originated outside the final 22, Van der Merwe's point haul in the match was a massive 48 points and he finished the competition as the top try scorer and the fourth-highest points scorer. After the Varsity Cup, Van der Merwe made his domestic first class debut for the in the 2016 Currie Cup qualification series. He made an appearance as a replacement in a 14–19 defeat to the, and made his first start a week later against the, which would be his final action in a Blue Bulls shirt.
2016–2017 : Montpellier
He moved to France in July 2016 to join Top 14 side, signing a youth contract with the team. He made his debut for the senior team in their Round 15 match against, replacing Joffrey Michel.
