Although his current favorite remains Batman, The Avengers are probably my son's second favorite comic-book heroes. I think he likes the ensemble nature of the concept. Spider-Man and Teenage Mutant Ninja Turtles are probably both in the top five, but the list gets fuzzier after these two.
We haven't read any Avengers comics together, but he has seen all of the animated Avengers we could find. These are roughly in order of my preference.
Avengers: Earth's Mightiest Heroes
Avengers: Earth's Mightiest Heroes (wikipedia, imdb, iTunes, amazon) is the most recent TV series at the time of this writing, this is my favorite of the available choices. There are two seasons of pretty solid episodes, although the last four or five episodes devolve into one world-ending threat after another (Kang, the Kree, Galactus) at a pace that feels rushed.
This is not an absolutely top-notch rendition of a comic book in animated form, but it's pretty solid. The writing, animation and voice-acting are good rather than amazing, but it doesn't feel dated, or like it's busy trying to waste your time.
Ultimate Avengers 1 & 2
Ultimate Avengers 1 (wikipedia, imdb, iTunes, amazon) and Ultimate Avengers 2 (wikipedia, imdb, iTunes, amazon) are movies, but similarly solid. The acting is a little more adult, the characters are more flawed (Hank Pym is a bit of a jerk in this series, and he and Jan fight quite a bit), and so it's a little less-well suited for my son's age, but it wasn't too bad.
Next Avengers: Heroes of Tomorrow
Next Avengers: Heroes of Tomorrow (wikipedia, imdb, iTunes, amazon ) is another movie that features the children of the Avengers after the Avengers are defeated by Ultron. It's not a true Avengers story for me, but it was well done, and better-suited for my son's age than Ultimate Avengers, so I was reasonably happy with it in the end.
Avengers: United They Stand
Avengers: United They Stand (wikipedia, imdb, iTunes) is a one-season TV show and is far and away the worst of this set, in my opinion. The characters are one-dimensional, I'm not fond of some of the voice acting, there are extended transformation sequences that just waste your time. Basically not worth watching unless you or your child has watched everything above and still wants more Avengers. My son enjoyed these, but I tended to tune it out.
He'll enjoy Joss Whedon's movie (wikipedia, imdb, iTunes, amazon) and its predecessors when I decide he's ready, but he hasn't seen any live-action comics yet, and I don't think that's going to happen very quickly.
There is apparently a new series starting, Avengers Assemble, which is the reason why Earth's Mightiest Heroes came to an end.