There are some really good things out there for enjoying these classics of Chinese literature.

Honestly, one of the things that sparked my love for Chinese history was the video game series Dynasty Warriors. Developed by KOEI, the game is a basic run-around, hack-and-slash built around The Romance of the Three Kingdoms. It's a super fun series. The most recent one to come out was Dynasty Warriors 8. (I don't think they've slipped a new one by me.)

I also have a couple of good movie suggestions. First, John Woo's Red Cliff (2008). It deals with the events surrounding the kingdoms of Shu and Wu teaming up to give Cao Cao a whooping. It has a number of great actors Tony Leung, Takeshi Kaneshiro, Fengyi Zhao and the visuals are just phenomenal. It is critical, however, that you get the two part original version, not the international watered-down version. The original is a total of four hours with subtitles, but I think your kid can handle the subtitles and it's not like you have to watch both parts back to back.

For the Water Margin, a good movie is Water Margin: True Color of Heroes (1992). It only deals with a particular story bit from the Water Margin (a.k.a. Outlaws of the Marsh), but it's a really fun movie.

However, those these are all great things, I would also suggest simply reading these stories with your son. These stories were initially written in serial format (like how comic books and tv shows are structured) so their great for doing little chunks at a time and picking them up later. Plus, as one of the oldest forms of human entertainment and history, there's a particular...magic that happens in the process of storytelling. It provides a particular interaction with the source material and understanding of that source material that just isn't quite matched in other forms of narrative telling. Of course, this could just be me getting all idealistic, too. Hah.

Anywho, I hope you get a chance to check some of these things out. A lot of my professional and non-professional interests started out because of doing things with my dad, so I hope your son gets inspired by your sharing of these things the same way I was.

You can give the manga Sangokushi a try; it's marketed as a "faithful adaptation of the Romance of the Three Kingdoms". It also runs for 60 volumes.

Also, Cao Cao did nothing wrong.

I'm quite fond of the anime Ikkitousen, which is loosely based on "The Romance of the Three Kingdoms."

Probably not entirely appropriate for kids, though.

I wasn't going to suggest it by when I saw others suggest a video game I thought I might go ahead. Suikoden 1 and two are great games for a kid around that age and are losely based on the long marsh.