Child Has Arm Numbness, No Pain. Any Ideas?

My 9 year old son has been saying his left arm goes numb every day, a couple or three times a day for about the past three or four months. There is no associated pain or pins and needles and there is no pattern to it - it can happen when sitting quietly, or out walking. He is very active, plays rugby, eats and sleeps well. He does not text but does play on the x-box, but this is limited to no more than 2 hours in any day.
He says when it happens it feels like if he were to pick up something, he would not be able to aim correctly, but would aim to the side of the object. |t is usually in less than a minute. Any ideas?