The child, named Techno Mechanius, goes by Tau and is the Tesla CEO’s 10th child with three women. The boy’s age has not been disclosed.
Musk, 52, dated the 35-year-old Canadian electro music star, who was born Claire Boucher, on and off between 2018 and 2022.
Walter Isaacson confirmed the couple had a third child in his biography Elon Musk, which is due to be released next week, according to a review in The New York Times.
The couple’s first child, a boy named X Æ A-Xii, or X, was born in 2020. In March, they announced the birth of a girl, Exa Dark Sideræl Musk, nicknamed Y.
The couple reportedly separated last year.
Excerpts released from Mr Isaacson’s biography have contained a series of bombshell revelations about the eccentric billionaire’s personal life and his involvement in the Russia-Ukraine war.
In an extract obtained by CNN, it was revealed that Musk had ordered SpaceX engineers to cut off Ukrainian access to Starlink satellites near the Crimean coast to prevent a surprise drone attack on Russian warships.
The drones packed with explosives “lost connectivity and washed ashore harmlessly” during the thwarted attack sometime last year. Musk reportedly feared the attack on occupied Crimea would amount to a “mini-Pearl Harbor”.
He later responded on Twitter/X, saying that the “Starlink regions in question were not activated. SpaceX did not deactivate anything.”
On 6 September, Mr Isaacson posted the first images of Musk with Neuralink executive Shivon Zilis and their twins, who were born in November 2021.
That photo prompted Grimes to accuse Musk of not allowing her to see their son X in a since-deleted post on Twitter/X.
“Tell Shivon to unblock me and tell Elon to let me see my son or plz [sic] respond to my lawyer,” Grimes reportedly wrote.
“I have never even been allowed to see a photo of these children until this moment, despite the situation utterly ripping my family apart.”
Mr Isaacson shadowed Musk for two years to gain an insight into every aspect of the billionaire’s world, according to publishers Simon & Schuster.
The author “attended his meetings, walked his factories with him, and spent hours interviewing him, his family, friends, coworkers, and adversaries”.
“The result is the revealing inside story, filled with amazing tales of triumphs and turmoil, that addresses the question: are the demons that drive Musk also what it takes to drive innovation and progress?”
The book is due to be released on 12 September.