I'm not sure that what you're asking is possible. An "explanation" is just a description of a deeper concept that implies what you're trying to explain. Physics is pretty much as deep as you can go. Put another way, physical theories are descriptions of the universe that agree with observation. You can explain other things in terms of physics, but it's hard to explain the nature of the universe in terms of something deeper without religion.

You might want to explain to your son how science works instead; how theories are formed, accepted, invalidated, and replaced. Looking up Feynman interviews on YouTube would be a good start.