Hollywood actor Robert Downey Jr — the real Iron Man — has officially named Sydney's Iron Boy, Domenic Pace, an honorary member of the Avengers.

Following a day of high drama that saw Iron Boy rescue Make-A-Wish Foundation journalist Hope Joy and defeat villain Ultron on the steps of the Sydney Opera House, Domenic received a special message from "Tony Stark", aka Iron Man.

Downey Jr, who plays the hero of the Avengers action movie series, filmed a personal message to Domenic congratulating him on his good work.

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"Hey Domenic, it's Tony. I'm a little undercover right now. I've shaved the beard, hat and glasses, because in case you don't know there was a civil war recently," Downey Jr said.

"Trying to keep it on the DL. Anyway, I've heard you've had quite the day. It's hard work saving the world isn't it?

"I hope you had lots of fun. I bet you enjoyed the helicopter ride this morning.

"I just want to wish you all the best. Keep up the good work.

"And listen here, I don't do this very often, I'm making you an honorary member of the Avengers. I hereby dub thee. Because guess what? Iron Man loves you."

Nine-year-old Domenic suffers from cystic fibrosis and was granted his dream to be Iron Boy for a day by the Make-A-Wish Foundation.

Iron Boy is awarded with a medal for his bravery, after defeating Ultron at the Sydney Opera House. ( ABC News: Lucy Marks )

Domenic Pace, aka Iron Boy, takes down a henchman on the shores of Sydney Harbour. ( Twitter/@MakeAWishAust )

Iron Boy defeats two of Ultron's agents at the Opera House. ( ABC News: Lucy Marks )