Thanks a lot for getting back to me it sounds about right as i have later found out he was on the trampoline in the garden and may have landed funny on that. You know what kids are like.
First thought is a pinched nerve. I get this in my left leg due to a herniated disc which puts pressure on my S1 nerve the main nerve that runs down my leg, he may have just fell wrong and his back is out of alignment sort to speak. Did he fall recently does he play sports etc? Again this is all uneducated guessing, as I am certainly not a doctor. Could be stress but at 7 yeas old I dunno.
Good Luck