Thanks for commenting, Izzie! One of my long term goals (which I realize is a lofty one) is to learn how to make even posed shots look as natural as possible, as though it's a candid image. However, this particular photo op was more just an exercise in working with available light, and my for year old happened to be the one willing to cooperate.
In other words, I wasn't after a fabulous portrait that day, but was wanting to work on using narrow lighting, and this was the best out of the bunch.
I noticed the difference between his eyes, too, but I am wondering if it would then look unnatural if the eyes appear the same shade, while one side of his face is bright and the other is dark? I'll play some more with it.