Hi, thanks, ***** ***** your query, will be replying in few seconds please. Dr. Yogindra
Please kindly send short video. A detailed neurologic examination by pediatric neurologist like muscle tone, jerks of the affected limb may give a diagnosis.
Particularly try to ascertain if he also has any numbness of affected leg like reaction to touch, hot or cold object, pinprick etc.
Jerks are by tapping on tendons at knee, ankle with rubber hammer. Other reflex is Babinski's. Please ask about these with your GP. If any joint has restriction of stretching, MRI test may be ordered.
You are welcome. Good luck to him.