Stepinac is assigned as the communications officer for Ranger station Charlie. He's temporarily the commanding officer of the station, in place of the absence of the head ranger Beaumont, who is currently out leading a mission, explaining why most of the rangers and soldiers stationed at the outpost are currently not available.
Later on, Ranger Beaumont and the other rangers stationed at RS Charlie would have already returned from their mission, however they are caught in an invasion of the station by a Caesar's Legion raiding party, who have either killed or captured everybody there. As Stepinac, at this point, is no longer found in the command building, it is presumed that he has been captured and enslaved or killed by the Legion. The booby-trapped bodies of NCR rangers and troopers, plus several holotapes documenting the invasion can be found in the aftermath of the attack.