How to be in a superhero movie at 12?

Best way for him to get into a superhero film is to simply go to where they're filming and watch. You may be lucky and they may want him as an extra. Many directors decide the shot will look better with a few people walking in the background ect. There is no way he will get a proper part in the film without any acting experience/qualifications.

Sign up for being extras. There are websites that helps become one. That way if filmmakers are directing a movie in your area, for example if Spiderman is being film in NYC and you live in NYC depending on the criteria that they're looking for and if the information you register with matches the criteria they will alert you and invite you to participate as an extra.

You can also look on craigslist to see if anyone filming a particular movie you're interested in. They will also ask for a specific criteria as well and then you can call them up.

For roles, you'll have to audience for it, go to acting school, film, and whatsoever.

Just tell your little brother that he'll have to work hard and look hard if he wants to be in a superhero movies. You'll have to compete for the role and work for it. Some people can audience and the role was given to them, but if they do a bad job they get replace. Even extras have to work hard if they ever wanted to take part in it and believe me there are stars that were once extras because they work for it.

He could audition for a minor character... Like the boy with the apple in Dark Knight Rises.