“When should I take my child to the doctor for stomach pain?”

Soon. Do not ignore.

It's a very open-ended question, but it depends on the severity of pain, the chronicity (how long it has been happening) and associated symptoms (e.g., if there is weight loss, anemia, blood in stool, abnormal bowel habits affecting ability to go to school or perform daily activities, etc.). More intense pain, longer duration and more associated symptoms would suggest that he should be checked out.

If it is chronic or severe or associated with fever vomiting or weight loss.