Sean Penn with Charlize Theron and her son Jackson
Sean Penn is adopting Charlize Theron's son.
The 54-year-old actor, who already has two children, Dylan, 23, and Hopper, 21, with ex-wife Robin Wright, has filed court documents in Los Angeles requesting to be legally recognised as the father of his fiancée's three-year-old child, Jackson.
A source told PEOPLE.com: "Charlize and Sean are very serious. Charlize says this is it. Jackson already calls Sean daddy. This is the first step toward them getting married."
The 39-year-old actress adopted Jackson alone when he was four-months-old in March 2012.
Another insider told Us Weekly magazine: "Sean always knew that to truly earn a place in Charlize's heart, connecting with Jackson was one of the most important parts."
While he is now in the process of adopting Jackson, the 'Milk' star's daughter, Dylan, recently admitted has also formed a close bond with Charlize.
Speaking in October, the young model said: "She's awesome. She's great and they're amazing together. As long as my dad's happy, I'm happy for both of them."
Asked what she liked about the blonde beauty, she added: "Well, she's brilliant for one thing, and she's the only woman since my mom who can shut my dad up. The only woman! That is a f**king challenge so... She's a powerhouse."
The couple only recently got engaged but started dating in late 2013, and have also talked about adopting more children together.
The insider said: "There is still talk of adopting again and giving Jackson a brother or a sister."