15 years old explanation

I will tell him to imagine he is an electron, which is rotating around me (the nucleus). I will draw a circular line around me in certain distance, and I would tell him that he can be rotating around me randomly, but he can’t be more than two seconds separated from the line.

After that, I’ll tell him to stop different times, showing him that the majority of the times I ask him to stop, he is standing at the line. Then I will explain that this line represent the bohr radius of the electron orbit around the nuclei, and even though the electron can be in any position, it will be mainly in this line.