Pitt adopted the 15-year-old in 2006.
While Brad Pitt and Angelina Jolie's children have been a focus of their divorce, it seems to be their oldest son, Maddox, is at the center of it all.
After news broke earlier this week that Jolie had filed for divorce from Pitt, reports surfaced that Pitt was being investigated for child abuse after Maddox allegedly inserted himself into a fight between his parents during a flight home from Europe last week. However, a source told ET it was "totally a child-parent disagreement," and any reports of inappropriate physical conduct are "exaggerated or untrue."
WATCH: What Angelina Jolie's Divorce Filing Could Mean for Her and Brad Pitt's 6 Kids
Maddox is the only one of his siblings to be born and adopted by Jolie pre-Brangelina. In fact, Maddox was adopted during Jolie's marriage to Billy Bob Thornton.
While Jolie and Thornton announced the adoption of 7-month-old Maddox, born Rath Bibol, from an orphanage in Cambodia together on March 12, 2002, only Jolie's name was on the adoption papers. The Wanted star filed for divorce from Thornton that July, asking for custody of then 11-month-old Maddox.
"Maddox was the first baby I ever held," Jolie told ET at the junket for Tomb Raider 2 in 2003. "He's really made me a woman and everything that is, you know, just the warmth and find that side of that very nurturing and open."
The actress continued to raise Maddox as a single parent until she and Pitt started dating in 2005. The pair first went public with their relationship in April of that year, while on vacation with Maddox in Kenya.
WATCH: Brad Pitt Seeking Crisis Management Amid Child Abuse Allegations and Angelina Jolie Divorce
In a 2007 interview with Vogue, Jolie credited her children, "especially Maddox," with speeding up her relationship with the Ocean's Eleven actor. "There was a coming together of him and Brad," she told the magazine. "It's a big thing to bring together a child and a father. It had never crossed my mind that Mad was going to need a father -- certainly not that it would be this man I just met. Until, of course, I got to know Brad, and realized that he is naturally a wonderful father. And we left a lot of it up to Mad, and he took his time, and then made the decision one day."
"He just out of the blue called him Dad," she said. "It was amazing… That was probably the most defining moment, when he decided that we would all be a family."
By that time, the couple had already begun the adoption process on Maddox's little sister, Zahara, in Ethiopia, adopting the 6-month-old in July 2005. On Jan. 19, 2006, Maddox and Zahara's last names were changed to Jolie-Pitt, after the actor officially adopted them as his own. The couple announced they were expecting daughter Shiloh just eight days earlier.
WATCH: How Brad Pitt and Angelina Jolie's Custody Negotiations Could Turn Ugly
Over the next few years, Maddox became a big brother four more times: to Shiloh in 2006, Pax in 2007, and twins Knox and Vivienne in 2008.
In December 2009, a now 8-year-old Maddox hit the red carpet, accompanying his parents to the Los Angeles premiere of Invictus.
The following year, Maddox was photographed getting in some quality father-son time with Pitt, as the two attended an NFL playoff game together.
WATCH: FBI 'Continuing to Gather Facts' Regarding Brad Pitt Child Abuse Allegations
Soon after, Maddox seemed to develop an interest in film, with the 11-year-old making a cameo appearance in his dad's 2013 film, World War Z, as a zombie.
"He gets shot in the head… multiple times. But he gets double tapped," Pitt joked to US Weekly at the time. "I don't know what that says about me as a parent."
"My boys think it's really cool," he told ET. "Finally a role they can see."
By 2014, 12-year-old Maddox appeared to have a bit more confidence on the red carpet, posing with his parents and siblings as he supported his mother's films, Maleficent and Unbroken.
The next year, it was back to movie sets for Maddox, who worked as a production assistant on his parent's film, By the Sea.
"We want to make it a family affair, and have the kids running in and out of set. And of course there's a few scenes where we had to make him work the parking lot," Pitt told ET at the movie's November premiere. "It's fun to see your boy grow up and take on responsibility. [It's] very nice."
After getting production experience on By the Sea, Maddox collaborated with his mother on another film, Cambodia, which is set to be released on Netflix later this year.
"Mad and I are now actually producing a film in Cambodia about his country and his history," Jolie gushed to ET in November. "We love working together."
WATCH: Angelina Jolie Pitt and Her Son Maddox Team Up to Direct 'Cambodia' Film for Netflix
In her divorce filing, Jolie has requested full physical custody of Maddox and his five siblings. However, a source told ET that Pitt wants to be with his kids.
"I am very saddened by this, but what matters most now is the well-being of our kids," he told People in a statement on Tuesday. "I kindly ask the press to give them the space they deserve during this challenging time."
Regardless of their split, ET's source says that Pitt thinks Jolie is a "great mom," and "he only has positive things to say about her as a mother."
"He will only say positive things about [the] mother [of his children]," the source continued. "He has also made it clear to friends to not say anything negative about her. He continues to hope that there is an amicable resolution in the interest of the kids."
WATCH: Angelina Jolie Opens Up About Her Marriage: 'Brad and I Have Our Issues'
