Every explanation gives something of a wrong impression. Keeping that in mind, we must try, or lose sight of it. St. Patrick used a shamrock. But of course, God is no object. To say 'Trinity' is to say that, in some way, God is a relationship as well as an organic whole.
Children usually have an easier time with mystery than adults do.
I think he might grasp it easier than you may think. Just click and go there. In fact, there's so much there that you'll have to bookmark it for repeated trips. I understood a lot about the origins of my faith from your Pentecostal History pages. That's why it's there.