He is the founder and the president of the “Association of Roma Friendship”, Luna, with the focus on building bridges and understanding between Croatia´s different ethnic groups through education.​After finishing his teaching degree at the University of Osijek, he has continued to work against stereotypes and prejudices in Croatia with the desire to improve the conditions for the Roma people. Kostic believes that the best way to reduce the gap between the Roma people and the other ethnic groups in the country is through education, and this is why he focuses on urging children to finish primary school and getting adults into work.