This could be "growing pains, " which is a loosely-applied term for mild, intermittent pain in growing kiddos.
It is poorly understood but thought to be related to excessive muscle exertion. It is benign and more worrisome to parents than serious. However, any limb pain
in a kiddo needs to be investigated so i would recommend seeing his doc for this just to be safe.
Answer assisted by David Miller, Medical Student