His name is Brian Breiter and he's a personal injury lawyer in Los Angeles. He tells us, he gets mistaken for the movie star all the time, but says he is NOT a professional look-alike.
As for the choking incident -- we're told Brian was eating at Big Pink Restaurant in Miami earlier this month when a nearby girl started choking on a piece of steak. Brian jumped into action and immediately performed the Heimlich maneuver, successfully dislodging the rogue piece of meat from the girl's windpipe.
The choking victim tells us she 100% believed it was Robert Downey Jr. the entire time he was rescuing her. It wasn't until later that same night she realized the truth ... when Brian friended her on Facebook.
But despite the disappointment, we're told the girl is still extremely grateful. She tells us if it wasn't for Brian ... she would have died.