What Causes Sudden Weight Loss And Severe Abdominal Pain?

My son has been having severe stomach pains since November. He has lost weight and can only eat very little. He is 47 years old. He recently had hernia surgery in April, but that has pretty much healed. He has seen a gastro doctor and a pc, and the surgeon who is also an oncologist surgeon. He has had a cat scan as well as bloodwork. He is scheduled for food allergy testing in about a week. Can you think of anything else it might be? The pain is in his lower left abdomen under the lowest ribs on both sides and also pain in his back which he thinks is muscular related.