Our oldest has crazy math instincts and I've often remarked to DH that he just knows it intuitively. He is younger than your DS, and most of the complex math we do is just for fun while driving, but i do sometimes encourage him to walk through his process and explain how he got there. I know it's a skill he'll need later on, and since it's very light-hearted, I think it also helps to draw him in a bit deeper into the magic (and method) of it all.