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I am Dr. Su S. and I will try my best to help you.
There can be a connection between headache & abdominal pain
There can be abdominal migraine.
As it is a new symptom, I would advise to get him an examination by doctor again to rule out any other cause.
I feel you can give him rizatriptan as the attacks are getting frequent.
Side effects can occur with any drug, but usually do not occur in all.
I hope this information will be helpful.
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Wish his healthy life.