I don’t see what connection he might be making between four-sided shapes in minerals and plants and animals. With minerals, distinct shapes like that emerge because they have a crystalline stucture at the atomic level that repeats throughout the material. While not always evident in the large scale, the crystal ‘habit’ of a mineral is determined by that atomic scale property.
Biological structures on the other hand are much more complex, and are typically the result of evolutionary pressures rather than simple physical properties. If the shape of some part of an organism enhances function and allows it to survive better, that trait gets passed down where less advantageous structures do not.
I don’t really know if that speaks to what he is asking about though. If you want something interesting to show him, though, check this out: The Valley of Square Trees. Not a lot of information in the article, but the fact that the trees in this valley grow square there, but not when grown elsewhere suggests something bizzare about the local conditions at play rather than an evolutionarily developed trait (which does of course contradict everything I said above and just goes to show the rules are not so rigid in biology as chemistry and physics).