Learning calculus helps improve your thinking skills. Most people I encounter on a daily basis could use some help in that department.
I occasionally pull out a calculus book for mental exercise myself (you can only do so much sudoko).
But calculus isn’t the only way to improve a child’s thinking skills. Most people I know who studied calculus way back when couldn’t even tell you what a derivative is anymore. So unless the kid is definitely bound for engineering or some physical science, the time spent learning calculus might be better spent learning something more applicable to his future avocation.