Sean Penn has filed papers asking to adopt Charlize Theron’s 3-year-old son Jackson, whom she adopted in 2012, a source tells People.

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“Charlize and Sean are very serious. Charlize says this is it,” the source says. “Jackson already calls Sean daddy. This is the first step toward them getting married.” Penn, 54, and Theron, 39, have been together for more than a year and were recently rumored to be engaged. Theron, who’s never been married, has no other kids; Penn has a 23-year-old daughter and a 21-year-old son with ex-wife Robin Wright.