The documents were filed in Los Angeles Superior Court on Wednesday by Federline’s lawyer, according to the source.
Spears and Federline’s divorce was finalized less than two weeks ago. At the time, the couple agreed to split custody 50-50 of Sean Preston, 22 months, and Jayden James, 10 months.
But a source close to Federline told PEOPLE that the would-be-rapper was worried Spears was exposing the boys to “unnecessary risk” and that his lawyer, Mark Vincent Kaplan, “hopes to get Kevin primary custody.”
In June, when the couple were wrangling over their divorce settlement, Federline was still resisting signing the papers over growing concerns about Spears’ erratic behavior.
Spears, who spent a stint at Promises rehab center, has been a fixture on the nightclub and party scene. The news of Federline’s request for primary custody comes as photos have surfaced of his ex-wife reportedly topless in a hotel pool.
Calls to Spears’s lawyer were not immediately returned.