Kids are generally wired to learn by doing. We can tell them how/when/why to shift, but they're really good at figuring things out by getting their hands on things and observing "What does this do?" It sounds like you've given him the basics, so now just encourage him to keep riding and keep shifting. As he learns by doing, add some coaching to help him with the details.
It's kinda like teaching kids to hit a baseball... With the smallest kids, we just want them to "keep their eye on the ball" and swing. As they gain experience, we can start talking about details like knuckle alignment, hip rotation, weight shift, arm extension, etc. If we tried to talk about those things with a t-ball player, we'd overwhelm him and take all the fun out of it.