If you have an amazon account, you can actually see a good chunk of the book for free here
Mouse over the cover page and click "look inside". I didn't count, but I think it gave me around ~30-40 pages of a 200 page book.
No problem! And thinking back, if you just want to show him more Chief, most of Halo 4 is safe to show. Though you may be wary of Cortana's physical appearance and the fact that she appears to be mostly nude.
The intro cutscene to Halo 4 is great and has some action, it even shows you that the Chief started off as a 6 year old kid, but at the same time it is kind of dark. Just be sure to DEFINITELY avoid the cutscene at 46:07: https://youtu.be/MlQeLoTUoZE
After combing through the Halo 4 cutscenes though, I am reminded of the mature storytelling so it might be a bit boring for a 5yo.
Halo 3 should be mostly safe, it's even kind of campy: https://youtu.be/8e1No_Oo8f8
You do have the Halo CE cutscenes too and they should be okay even with the Flood, but they're kind of dated. I'll link them anyway: https://youtu.be/32wp-gaRX8Q
Lastly, I was thinking over Halo Legends, and all of them should be okay minus "Homecoming" and "The Duel". PROTOTYPE is super action oriented but does have some blood and gore.
Also, what just crossed my mind is Halo: The Fall of Reach Animated Series, which is on Netflix. That should be okay for your son. It's a lot like Ender's Game if you've seen the movie/read the book.
Last but not least, a Scholastic Halo Guide was recently released called The Official Spartan Field Manual. It's meant for most ages, but I think your son would find the pictures and general feel of the book pretty cool. Here's an Amazon link to it: https://www.amazon.com/HALO-Official-Spartan-Field-Manual/dp/1338253638