A 4-year-old boy presents to your office with watery nonbloody diarrhea for the last three months. He has occasional abdominal pain prior to the onset of the diarrhea and his mother noted worsening flatulence, especially after meal times. Mom denies any recent travel or camping and the patient does not go to school or daycare. On exam, he is 60th percentile for weight and height (similar to his last well-child check), appears well, and has a normal abdominal exam. What is the most likely diagnosis