[clarification needed] Mattheus was born in Port Elizabeth, where he attended Grey High School.
[4] He played off the bench in a 17–13 victory over Free State U19, before starting four of their final five matches of the competition, contributing 18 points with the boot.
He joined the Eastern Province Kings academy on a full-time basis for 2015 and started the Under-19 team's opening match of the 2015 Under-19 Provincial Championship season, a 28–15 victory over Free State U19.
[10] Mattheus also played off the bench in their 31–15 semi-final victory over Free State,[9] as well in the final, which Eastern Province won by beating the Blue Bulls 25–23 in Johannesburg[11] to win the competition for the first time in their history.
[12] In 2016, Mattheus was included in the Eastern Province Kings squad that played in the 2016 Currie Cup qualification series.