A 7-year-old boy presents with a 2-week history of intermittent periumbilical abdominal pain, worse in the mornings causing him to miss school, associated with occasional loose stools but no blood. There is no weight loss or nocturnal waking. Examination is unremarkable. His mother is very anxious about the symptoms. According to NICE NG133 (Irritable bowel syndrome in adults - principles apply), what is the most likely diagnosis?