Benjamin Keough's cause of death has been confirmed by the Los Angeles County Medical Examiner's Office.
The 27-year-old, who was the son of Lisa Marie Presley and grandson of the late Elvis Presley, was found dead in Calabasas, California, on Sunday.
After an autopsy, his cause of death was listed as a shotgun wound and his manner of death was listed as suicide.
Following his death, Lisa Marie's manager said she was "heartbroken, inconsolable and beyond devastated".
"She adored that boy.
He was the love of her life," Roger Widynowski said, adding that she was "trying to stay strong" for her three daughters.
Benjamin was the younger of the two children Lisa Marie Presley had with her first husband, musician Danny Keough, before their divorce in 1994.
The other is Mad Max actress Riley Keough, 31.
Lisa Marie had previously described Keough's resemblance to her famous father as "just uncanny".
Footage of Keough and his siblings featured in the 2012 music video for Lisa Marie Presley's posthumous "duet" with Elvis, I Love You Because.
Her close relationship with her son, whose middle name was Storm, also apparently inspired the title track on her third album, Storm & Grace, released in 2012.
He reportedly signed his own record deal with Universal in 2009, although no music was ever released.
In 2013, Lisa Marie told the Huffington Post she was letting Keough do "his own thing".
"I'm going to let him decide when he wants to go out and do what he wants to do," she said.
Lisa Marie also has twin 11-year-old daughters with her fourth husband, musician and producer Michael Lockwood, who she wed in 2006 after brief marriages to pop star Michael Jackson and actor Nicolas Cage.