Story highlights 19-year-old Quintonio LeGrier was shot dead by a Chicago police officer

His father is suing the city, saying "inexperienced officers" snuffed out his son's future

Police called the teen "a combative subject"; his family says he faced mental health issues

(CNN) Antonio LeGrier hoped police would help his son when he called 911 last week.

Instead, an officer shot 19-year-old Quintonio LeGrier dead inside a Chicago apartment building.

"I never once thought that when he entered that staircase, that his life would be ended by someone who didn't know what to do," Antonio LeGrier said, crying as he described his son's final moments to CNN's Brooke Baldwin on Tuesday. "Now my only son is gone, and I don't know what to do."

The father said he called out for help after the shooting but never saw police or paramedics provide medical aid to his son. Then, he said, police held him for questioning for hours after the shooting, not telling him whether his son, an engineering student at Northern Illinois University, had lived or died.

"I feel like I was robbed of everything," he said.