ORLANDO, FLORIDA - Seven-year-old Alex was born with a partially developed right arm, making it difficult for the young boy - who loves superheroes - to ride his bicycle.

Hollywood superstar Robert Downey Jr., known to legions of fans across the globe as Iron Man, did a special meet-and-greet with the shy Alex - and also delivered a 3D printed bionic arm in a special "Stark Industries" case to the surprised boy.

Their sweet encounter was captured in this video, which Downey later posted to his Facebook page on Thursday, calling Alex "the most dapper 7-year-old I've ever met".

The arm was provided at no cost to Alex's family, courtesy of Albert Manero, a student at the University of Central Florida and the founder of Limbitless, a company which builds such limbs for disabled children whose families otherwise cannot afford them. Manero is part of a group of students recognised by Microsoft's OneNote team's Collective Project campaign, reports USA Today.

"Do you know who that is?" Alex is asked during the video.

"Iron Man," Alex says with a grin, as Downey pumps his fist in the air.

"Half the time, I design one of these, and it keeps breaking on me," he tells Alex in character as the billionaire inventor Tony Stark, Iron Man's alter ego. "What I do is I keep on working on it, like you're working on it with Albert."