Sean Penn and Charlize Theron are ready to take the next step in their relationship — and it involves the actress’ 3-year-old son, Jackson.

Sean “has already begun the paperwork to formally adopt Jackson. Charlize never placed any pressure on Sean to adopt Jackson. It has been a very natural process of them becoming a true family,” a ‘Radar Online’ source reveals.

charlize theron

Charlize and Sean with Jackson.

The pair, who have been dating for about a year, reportedly got engaged in November. Charlize allegedly began talking about marriage after seeing how her Oscar-winning beau interacted with her son, whom she adopted from South Africa in March 2012.

“Jackson calls Sean ‘Daddy,’ and Sean just lights up around him,” the insider continues. “Sean picks Jackson up from nursery school, and all of the parents know him.”

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The 54-year-old actor is already dad to daughter Dylan and son Hopper from his previous marriage to Robin Wright. He was also previously hitched to Madonna from 1985-1989. Though never married, Charlize was formerly in a nine-year relationship with actor Stuart Townsend.

“For the longest time, Charlize thought it was just going to be her and Jackson, and she was ok with that,” the ‘Radar’ insider adds. “There could be a sibling for Jackson in the future as Sean and Charlize have talked about adding to the family.”