I would start with simple chores, anatomy, safety, and things like that. It's important for children to learn early on that horses aren't machines and can be dangerous, etc.
For learning colors and anatomy, some of my old instructors used to say "What color is (horse's name)" and we would all guess. She would say if someone was right or if we were all wrong she would say what the color was, and then come back to that horse again later. Anatomy, it would be almost like a command class sometimes (touch the horse's poll) and other times it would just be questions while we were doing something else, like trotting.
When he first starts to ride, it might be a good idea to put him on a lunge line so he can start to learn about and try to achieve correct posture early on.
Also, make sure that you have a helmet he could borrow (that could fit him reasonably well) or that his parents know they have to get him one. I don't know about your state, but in mine minors have to wear a certified horseback riding helmet, a bike helmet is a no-go.
Hope I helped!