The Iron Man star, 49, who battled drug addiction for years before getting sober in 2002, has rallied to help his son Indio, 20, who was arrested for cocaine possession on June 29.
A family source says Indio, a musician, has struggled with substance-abuse issues in recent years. He voluntarily checked himself into rehab after the arrest.
"Robert clearly understands the struggle, and while of course this relapse was very upsetting to him, he is very proud of his son," the source tells PEOPLE in this week's issue.
Indio has the support of his family, including his mom, Deborah Falconer (who was divorced from Downey in 2004), and Downey's wife of eight years, Susan. (The couple are parents to son Exton, 2 ½, and are expecting a daughter in November.)
"Indio truly wants to stay clean," says the family source. "And no matter what, his dad will be right there with him."
For more on Robert Downey Jr. and Indio, pick up this week's issue of PEOPLE, on newsstands Friday.