Kids get RLS (Restless Leg Syndrome) that can start at a very earl age. Many people that had growing pains as kids ended up with RLS. I did.

RLS can have many symptoms beyond just the need to move one's legs and/or feet. Pain of varying degrees and kinds can be a symptom too. If movement of the affected leg or foot eases the symptoms it might be RLS. There are no clinical tests for RLS. It's symptomatic. And they do use sleep studies.

Also, everyone knows that one side of their body is bigger than the other. Is it possible his shoe fits a bit tighter on the affected foot and is putting his foot to sleep or pressing on the nerve on top of the foot (called the instep, located between where the foot meets the leg, right below the ankle, midway to the top of the toe joints)?