Hello & welcome to Just Answer! If you have any questions after reading my answer, be sure to let me know.
Incredible amount of pain is NOT a sign of regular stomach flu.
This can suggest a serious cause like appendicitis, severe infection, inflammation of the gut or the surrounding abdominal structures
I would suggest that you need to look at taking him to urgent care to find the cause of the symptoms
so that we do not miss a serious cause of the present symptoms
I would not advise trying to mask the symptoms by giving medications other than tylenol at the present point of time,
Do get back for any clarifications
Dr Gupta, MD