What Causes Frequent Episodes Of Weakness In Leg?

Hi - my 12 year old son has been experiencing weakness of the legs at least once a day for the past two weeks. For the past two days it has been several times a day. He needs to sit down and it passes. He had a bad sore throat over three days about a week a go but that has now cleared up. Went to GP a few days ago who tested blood pressure and heart and said it would probably clear up but it hasn't. Any ideas?