What Causes Pins And Needles And Pain In Legs In Children?

My son had just turned 3 and complains very often of pins and needles in his legs, he describes it as ants walking in his feet, it happens on a daily basis, usually after sitting on the loo or sitting in his car seat. Sometimes he can t stand on his legs and will just fall down when trying to stand and complains of pain in his leg, do you have any idea what can cause this?