A spokesperson for Lisa Marie - who also has daughter Riley with Danny and 11-year-old twins Finley and Harper with her fourth husband Michael Lockwood - said: "She is completely heartbroken, inconsolable and beyond devastated but trying to stay strong for her 11-year-old twins and her oldest daughter Riley. She adored that boy. He was the love of her life."
Benjamin was known for his strong resemblance to his late grandfather, Elvis Presley, which his mother has admitted left fans shocked.
She previously said: "Ben does look so much like Elvis. He was at the Opry and was the quiet storm behind the stage.
"Everybody turned around and looked when he was over there. Everybody was grabbing him for a photo because it is just uncanny."
Riley Keough also admitted a "giant hole" has been left in her heart following the death of her brother Benjamin.
The 31-year-old actress wrote: "Mornings are the hardest. I forget you're gone. I can't cry because of the fear that I will never stop.
"A pain that's new to me. You. There are no words for you.
"Angel is the closest I could think of. Pure light. Baby brother. Best friend. Wild man. Intellectual. Witness to my life. Twin soul. Protector. Too sensitive for this harsh world.
"I hope you give me strength to endure the giant hole you've left in my heart. I hope you give me the strength to eat. I hope you're cradled in love.
"I hope you feel my love. I hope you feel god. You are god. I can't believe you've left me. Not you sweet Ben Ben. Anyone but you.
"I guess this is true heartbreak. I hope we meet again."
WHERE TO GET HELP:
If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.
OR IF YOU NEED TO TALK TO SOMEONE ELSE:
• LIFELINE: 0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
• SUICIDE CRISIS HELPLINE: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202