Robert Downey Jr. Helping Son with Addiction -- Indio Downey's Drug Problem

Robert Downey Jr. has been very privately helping his son struggle with addiction problems for years ... with some success ... sources connected with the family tell TMZ.

TMZ broke the story ... 20-year-old Indio was arrested Sunday for cocaine possession. We're told it's the latest slip in his ongoing struggle with drugs ... and his dad has been deeply involved in Indio's treatment.

Indio has been in and out of treatment programs for several years. One source says until Sunday Indio had a fairly long stretch of time when he was clean, in no small part because of Robert's counseling.

We're told Robert feels a degree of guilt as a father ... because he's been through the same struggle.

One source put it this way, "Because of Robert's own struggle he's very much in tune with his son's sobriety. And that has really helped Indio."

Everyone we've spoken with says Indio is a good person who is polite, well-mannered and serious about trying to stay clean, but like so many people with addiction problems, he's slipped.

Indio has also seen Robert help others who have struggled, including Mel Gibson.