What Causes Severe Leg Pain At Night?

my 16 months old son suffers from leg pains at night.he is a very active child.i have noticed that the pain occurs usually when he runs more than usual during the day.I have consulted several doctors,they have done ultra sound of hips and xray. they say everything is alright,its "growing pains". i am worried about recurring nature of the pain. plz advise.