Academy Award-winning actor Sean Penn has reportedly filed formal papers to adopt the 3-year-old son of longtime girlfriend Charlize Theron.

People magazine reports the 54-year-old Penn and the 39-year-old Theron are "very serious" about the move, and the young boy "already calls Penn daddy."

"This is first step toward them getting married," a source from People said.

A fellow Oscar winner, Theron adopted Jackson, then a 4-month-old baby, in 2012; he is her only child.

"Sean always knew that to truly earn a place in Charlize's heart, connecting with Jackson was one of the most important parts," a source reportedly close to Penn said.

Penn, meanwhile, has a 23-year-old daughter, Dylan, and a 21-year-old son, Hopper, from his first marriage to actress Robin Wright.

Though they have remained largely private about their relationship, Penn and Theron have been one of Hollywood's hottest and highest profile couple since they began publicly dating more than a year ago.

"Charlize likes guys with more substance," a source said. "She finds Sean very charming, and he makes her laugh. She definitely has him wrapped around her finger."

According to Us Weekly, Theron and Penn are also talking about "adopting again and giving Jackson a brother or a sister."

Penn has won Academy Awards for his portrayals in the drama "Mystic River" and the biopic "Milk." He is also noted for his work in such films as "Carlito's Way," "Dead Man Walking" and "I Am Sam."

Meanwhile, Theron walked away with an Academy Award for her role as Aileen Wuornos in the 2003 film "Monster." The South African-born starlet is also revered for her work in such critically acclaimed films as "The Devil's Advocate," "The Cider House Rules" and "Mighty Joe Young."

Reps for Penn had no comment about the couple's rumored actions.