The Hollywood actor and his producer wife Susan welcomed their first child together in February (12), and Downey, Jr. admits he struggled to enjoy the excitement of becoming a parent again because he harboured so many worries for his new son.
But his fears disappeared once he laid eyes on little Exton - a half-brother to Downey, Jr.'s teenage son Indio, from his previous marriage to actress Deborah Falconer.
He tells Esquire magazine, "There was all this trepidation, all this projection, all this anticipation and goodwill and a good vibe about it. But what you're squeezing to the side - or what's in the glove box - is these thousands of forms of fear.
"And then he was born and they've all just kind of scattered now. It seems like he's always been here."
Now he is a dad again, Downey, Jr. insists he won't be trying to act like an Iron Man-esque superhero in front of his baby boy - because he wants his son to grow up as well-rounded as possible.
He says, "Do I want to be a hero to my son? No. I would like to be a very real human being. That's hard enough.
"Every dad casts a shadow, you know? And that shadow is you're disappointed, you're resentful, or you feel so supported and loved you don't understand why life is so hard anyway - or, you know, it's so long and so dark that you can never step out of it, so you might as well not even try."