YEARS after con­quering his own drug demons, “Iron Man” star ROBERT DOWNEY, JR. is caught up in a new rehab drama – his 20-year-old son Indio is un­dergoing drug treatment.

“This is a nightmare for Robert because his boy has never been in trouble be­fore,” said an insider close to Downey, 48. “He’s praying for Indio’s recovery.”

Indio sought treatment af­ter taking prescription pain medication, said his mother, Downey’s ex-wife, Deborah Falconer.

“He was not addicted,” Falconer told The ENQUIRER. “He was taking one pill a day.”

She confirmed that Indio – an accom­plished musician – is in a “treatment facility” where he’s receiving “holistic, natural and orthomolecular” therapy.

“We are hoping to do it the ho­meopathic way rather than with medications,” said Falconer, an actress and singer. “He’s being treated with vi­tamin supplements and organic foods.”

Meanwhile, In­dio’s career has been flourishing. His rock band “The Seems” has played at L.A.’s famed Viper Room and recently inked a deal with Warner Bros., according to MTV.

The young man has also worked as an actor, and, ironically, appeared in a drug movie titled “Run for Roxanne,” due out in 2014.

But for the “SHERLOCK HOLMES” star Downey, his son’s stint in rehab is reminiscent of his own headline-making strug­gles with drugs. Downey – recently named by “Forbes” magazine as Hollywood’s highest-paid actor – once admitted that he’d been addicted to drugs since the age of 8, and he spent his early 30s locked in a nightmare of substance abuse.

He married Falconer in 1992, but after he was ar­rested for drug possession, DUI and possession of a concealed weapon in June 1996, she left him and took Indio with her. Two more drug busts soon followed, and Downey threatened to kill himself before being giv­en three years’ probation and a three-month sentence in rehab. In 1997, he fell off the wagon with a wild crack cocaine spree and his probation was revoked.

He was slapped with a six-month jail sentence on the probation violations. By 1999, he’d been in and out of rehab and was back on drugs and liv­ing in hotels. At one point, he sought help at three different rehab clinics in one week and ended up in a hospital psych ward.

In August 1999, he landed in the California’s Substance Abuse and Treatment Facility and State Prison, Corcoran for repeated parole viola­tions and spent a year behind bars.

He was arrested twice more on drug charges and spent more time in rehab before finally getting sober for good in 2001.

The talented actor rebuilt his ca­reer and married producer Susan Levin in 2005. Their son Exton was born in February 2012.

As for Indio, Falconer often talks to him on the phone and told The ENQUIRER: “He’s doing great. He’ll be back playing music and do­ing better soon.” And a source close to Downey told The ENQUIRER that Indio’s condition “doesn’t appear to be anything serious,” adding that he “does not have a problem with prescription drugs or narcotics.”