Kojić's first appearance in international competition took place when he ran in the second heat of the boy's 800 metres at the 2011 World Youth Championships in Athletics, running a time of 1:54.97 (min:sec).
Just a few weeks later, he finished the 800 metre in second place at the 2011 European Youth Summer Olympic Festival, recording a time of 1:51.55.
A year later, Kojić ran the men's 800-m race at the 2012 World Junior Championships in Athletics with a time of 1:54.79.
In December 2012, he was named "Male Sportsperson of Čačak" after successful indoor and outdoor track seasons that year.
Only a week later, at the 2013 Mediterranean Games, he ran the first heat of the 800 metre and recorded a personal best time of 1:49.10 in the 800, even beating Giordano Benedetti.