It happened around 10 p.m. Saturday at the intersection of US Highway 401 near Club Pond Road in Raeford.
Christman was a passenger in the truck driven by his friend Christian Martinez when the firework came in the window and exploded.
Flames quickly spread through the cab. Martinez was able to get out the driver's side door, but Christman was trapped when the plastic of the passenger side door melted it shut. He was able to escape before the truck exploded.
"We heard it was called a mortar, firework ... it was described as like a display style of firework and it hit right beside our son's legs and at that point it started sparking and going off," Christman's father Brent Christman said. "The truck filled with smoke. Alex's friend, Christian Martinez was able to get out of the truck Alex was not able to get out right away the door was locked and as the truck filled with smoke he ended up having to dive out of the window of the truck."
Christman is now undergoing treatment at the Jaycee Burn Center in Chapel Hill. His parents flew down over the weekend to be at his bedside.
"He has burns mostly on the bottom part of his legs, some on his forearms, the worst burns that he has are towards his feet, tops of his feet and his toes," Brent Christman said.
Hoke County investigators haven't said if they have any suspects in the attack.
The men were only able to tell police they saw an older model white sedan before the attack.