Instead of suggesting what to do next, ask, "The fuel levels are low; what is our backup plan?" This forces him to problem-solve within the narrative.
Challenges like a "bridge collapse" or a "lost map" allow him to take the lead in navigating a crisis.
At this stage, play moves beyond "what things are" and focuses on "what things can be." A simple cardboard box isn't just a container; it becomes a cockpit for a mission to Mars or a deep-sea submersible. This shift demonstrates a boy's ability to hold multiple layers of reality at once—the physical reality of the room and the mental reality of the story.
When joining a boy in mature play, the goal is to be a "supporting character" rather than the director.