J.K.
My son had this pain as well. I took him to the Dr. who refered me to a pediatric foot specialist at Children's hospital.
It was caused by his lack of muscle development in his arch and calves. It is completely normal (and often diagnosed as "growing pains") and once we got proper shoes with arches, he was MUCH better. We also had the option of getting insoles.
I had been buying the most expensive shoes and not paying attention to the arch support. Most often, it is the cheap shoes that have the best arch support, oddly enough.
Good luck!
J.