I'm going to move your topic to the physics forum so that our Experts can help you there.
I think the first step to making the world better by using natural resources is to make sure that solar energy is useful to us. We have a simpler experiment on this website that involves constructing a simple circuit and measuring how much electricity the solar panel creates in different lighting conditions. It's still a bit advanced for a student in K-5, but it looks like it can be simplified so that he doesn't have to know the equations, just qualitatively what they mean.
Here's the project: http://www.sciencebuddies.org/science-f ... p014.shtml
With that project, his question could be narrowed down to how much electricity is created depending on the intensity of the light. So really, he'd be figuring out how the sun's energy can be transformed into electricity.
You could even look at how much electricity household items use, and compare that with the output of the solar cell.
Good luck and let us know if you have more questions!