Looks like a group of Yugoslav Army privates, which could be from 1919 or early 1920s. Those pocket flap shapes were also used on the uniforms of the Austro-Hungarian Army during WWI, but the hats are definitely Yugoslav; it's a traditional Serbian hat called a Šajkača. I think the photo I linked is from a bit later, but the uniforms and hats are similar to those from the early 20s.

The Kingdom of Serbs, Croats, and Slovenes (the precursor to the first Yugoslav republic) experienced a series of border conflicts and internal ethnic conflicts in the first part of its existence and mobilized a pretty large army in 1919-1920, so it wouldn't be surprising if he had been drafted and then demobilized without seeing combat.

Hello there! As your question is related to looking for identification/information regarding military personnel, our Guide on Military Identification may be of use to you. It provides a number of different resources, including how to request service records from a number of national agencies around the world, as well as graphical aids to assist in deciphering rank, unit, and other forms of badges or insignia. While the users here may still be able to lend you more assistance, hopefully this will provide a good place to start!