Cannon's brother also was in the car. He grabbed the wheel and sped away.
The shooting happened at about 9:15 p.m. Saturday at 105th Street and Peoria in the South Side's Washington Heights neighborhood.
The teen's murder means another family and another community are now in pain. The 17-year-old Chicago student was a fixture in his neighborhood, and Saturday night, he became the latest victim of gun violence.
"The odds are terrible against young brothers. It's like you are an endangered species," said uncle Travis Williams.
Still touched by shock and disbelief, Williams was doing what he could Sunday to deal with the murder of his nephew.
Cannon was shot in what appears to be an attempted robbery.
The tragedy comes as his family says he considered a career in the military, which is something his uncle encouraged because of the continued gang and gun violence claiming young lives on Chicago's streets.
"Even away in the war right now, your chance of knowing your enemies is better over there than surviving walking around these streets of Chicago with all the stuff going on," Williams told ABC7 Chicago.
Investigators say the attack happened just after Dequarius, or "D" as his friends called him, and one of his brothers visiting from Iowa waited in a car near for a friend. That's when witnesses tell police an unidentified person approached the car on foot, demanded valuables from the duo and before they could respond, and simply shot Cannon, who was in the driver's seat, in the head.
Cannon's brother then grabbed the wheel and sped back to the family's home where relatives tried to revive the teen. He was pronounced dead at an area hospital a few hours later.
Sunday, a community pleaded with whoever was responsible to come forward.
"The only thing to tell them to do is to do the right thing and turn yourself in, while you still have a chance, because there are some people that really loved this young man," said Pastor Torrance Garner, a family friend.
Dequarius Cannon lived with his mother and grandparents, and those who knew him say he was always there to help a friend.
His family remembers his love of music and thirst for life, as they struggle with his death.
"I'm still trying to fathom the fact that my nephew is gone," Travis Williams said.
Calumet area police detectives are investigating the attack and are still hoping for a lead that will break the case and bring about an arrest.