My son is about 13 years older than your son. He surprised me one day when he wrote his college entrance essays and explained why he wanted to become an engineer. I'll never forget one section. He said, "one day my dad brought home a bag of batteries, light bulbs, motors and wires. He showed me how to make a circuit to light up a bulb with the wires and battery. From that day on, he knew there was an explanation to how everything worked. Before that day, a flash light was a mystery and magical."...and I'm enjoying this site~~ thanks for making it a fun 'first' post
I never knew it made such an impact. I hope your son remembers this experience as well.
Back to your question...
You can describe the battery like a car (or bicycle) running fine on a flat road (nothing connected to a battery). Then the light bulb is like a small hill that slows the car slightly and the motor is like a bigger hill that slows the car or bicycle more.
You can also try talking about the three bears story and then ask him what he thinks is happening to the battery with nothing, the bulb and the motor - then see how he describes it.
By the way, thanks for sharing this story with us, too.