Stomach pain diarrhea in child after hernia surgery

My son age 6 had inguinal hernia surgery 1 mo. ago. Everything healed well and we had no more problems.  About 2 weeks after surgery he started having horrible stomach pains usually during meals- where he dry heaved, and had horrible diarrhea that lasted a couple hours.  Then it would all go away as fast as it started.  Havent found any reason/food that is causing this.   This was happening every couple of days.  He lost 4lbs in 3 weeks.  Although it seems to be better now, he still complains that his tummy hurts alot and still isnt eating well.  He had stool cultures which came back normal.