The younger Downey also faces a misdemeanor charge of possession of drug paraphernalia -- a smoking device -- a spokesman for the L.A. County District Attorney's Office told L.A. Now on Thursday.
It all stems from Indio Downey's June 29 arrest in West Hollywood after police said he was seen on a Sunday afternoon "apparently using" a pipe to smoke drugs in the passenger seat of a car.
After the arrest, it emerged via TMZ that Robert Downey Jr. -- who has rebounded rather spectacularly in recent years from his own decades-long struggle with substance abuse and legal problems -- had been privately helping his eldest son fight addiction for years.
Indio Downey's mom is Deborah Falconer, the actor's first wife, who was married to the Oscar-winner during his numerous drug-related arrests and his prison stint. She filed for divorce in 2001 after RDJ was arrested yet again and then fired from his supposed comeback role in "Ally McBeal," and took their son with her.
Robert Downey Jr. finally cleaned up his act before marrying Susan Levin in 2005. They welcomed a baby boy, Exton, in 2012 and are expecting a girl this November.
Indio Downey, who sings and plays guitar in L.A. band the Seems and has a couple of movie credits, will turn 21 in early September. He remains out on $10,250 bail and according to L.A. Now will be arraigned later this month.
TMZ also noted that because Indio Downey would, if convicted, be a first-time offender, he'd likely be a candidate for a drug-diversion program rather than jail time.