What Causes Extreme Pain In Left Side Of The Body?

My 6yr old son suffered extreme pain in his left side on Sunday night. This is the 4th episode of this in the last 8months. I called a doctor to our house after hours and he said he thought it was a hernia. I brought him to A&E and they found no hernia. They did an x-Ray of his hips and an ultrasound on his kidneys and found nothing. No UTI either. The pain is so bad he cannot walk when it happens. Any ideas what this could be??