After his arrest for cocaine possession on June 29, Indio Downey, the eldest child of movie star Robert Downey Jr., drove to a Malibu facility where he voluntarily checked himself into rehab for substance abuse.

A family source says the actor has his son's back 100%. "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 reveals.

The 20-year-old musician has reportedly struggled with addiction for quite some time and Robert — who battled his own drug abuse for years — questioned whether his look-a-like son had inherited that dependence.

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But with the Iron Man star and Indio's mother, Deborah Falconer (who divorced Robert in 2004), standing behind him, the young adult has a solid support system.

"Indio truly wants to stay clean," the family source tells People magazine. "And no matter what, his dad will be right there with him."