Iron Man actor Robert Downey Jr has joined his Avengers co-stars Chris Evans (Captain America), Chris Hemsworth (Thor), Mark Ruffalo (Hulk) and Tom Holland (Spider-Man) in sending a message of encouragement to six-year-old Bridger Walker (below).

The boy was mauled and needed 90 stitches from an emergency surgery, after protecting his four-year-old sister from a charging dog.

Walker, whose family lives in Wyoming in the United States, put himself between a German Shepherd mix and his sister on July 9.

In true Tony Stark fashion, Downey Jr promised to go one up on Evans' promise of a Captain America shield as a gift.

"Bridger, you're a rock star," Downey Jr said in a video message shared by the boy's aunt Nicole Walker, who had earlier posted about the incident on Instagram and tagged several Avengers stars. Her first post went viral and led to an outpouring of support online for Bridger.

"I hear he's sending a shield your way. I'm going to do one better. You call me on your next birthday. I got something special for you. By the way, that's a promise. A promise beats a shield."

There have many messages of support from around the world for the brave little boy, who was praised for his selfless act. Other actors who have played superheroes, including Anne Hathaway, who played Catwoman, and Hugh Jackman, who played Wolverine, have also given him the thumbs-up.