The La Salle College student secured his place in the 24th International Athletics Championship when he won three medals at two recent events.
For a place in the state team, competitors were required to win a medal in either challenge.
Seth scooped silver in the WA Little Athletics multi-event in February.
In last month’s State Championships, he took bronze in the 90m and the 200m hurdles.
The 14-year-old, of High Wycombe, faced his toughest call in the multis, competing in five track and field events as the mercury soared to 41C during the two-day challenge.
Coaches Neil Wilkinson (long jump), Dennis Fare (hurdles) and Jade Lewis (endurance) helped prepare Seth for the competition.
Seth’s mother and sprint coach Nicola Carvalho said her son’s athletic ability blossomed under the guidance of Carlisle Rivervale Little Athletics Club and hurdle coaches Steven and Dennis Fare.
“Seth started Little Athletics at Under-11s with Forrestfield Little Athletics Club about four years ago,” she said.
“He always had a natural talent at a young age, fostered by his commitment and the assistance of his coaches over the years.”
Seth averages nine to 10 hours of weekly training over three days.
“I was Seth’s coach for hurdles and sprints at the beginning, but as he got stronger in hurdles, I needed to source another coach with a little more knowledge,” Ms Carvalho said.
Hurdles clinic Jason Moyle Elite Athletics invited Seth to participate in its training program last year.