Hello, I carefully read your message and understood your medical issue. Left lower abdominal pain is quite common among the people who are not used to physical effort or sports. They are considered as side aches or cramps and by the time you get used to exercising they do not appear anymore. Don t let your son drink too much fluids before running schedules and avoid gas-producing foods. To help your child relieve himself when experiencing this after running tell him to breathe deep and slowly then exhale fast. Don t let him do excessive physical efforts that he can not tolerate. Considering your situation I think we must take into account any appendicitis inflammation of atypical symptoms. So if the pain appears too often please see a doctor. Above all, there is nothing to worry much. I hope this was helpful for you. Dr.Albana