Having grown up in Wellington in the Western Cape province in South Africa, Van der Westhuizen was selected to represent Border at the annual Under-16 Grant Khomo Week competition held in Upington, where he scored a try in their match against the Pumas.

[8] In 2014, Van der Westhuizen was included in the South Africa Under-20 squad that participated in the 2014 IRB Junior World Championship held in New Zealand.

[11] He played off the bench once again for their final pool match, a 21–8 victory over Samoa as South Africa finished top of the group to set up a rematch with New Zealand in the semi-finals.

Van der Westhuizen started the semi-final and helped South Africa secure their fourth consecutive victory over New Zealand at this level, winning 32–25.

[13] Van der Westhuizen returned to domestic action a month after the Junior World Championship, making ten appearances for the Blue Bulls U21s during the 2014 Under-21 Provincial Championship and scoring a try against the Free State U21 team[14] as they reached the final of the competition, with Van der Westhuizen coming on as a second-half replacement to help the Blue Bulls win the title by beating Western Province U21 in the final.