Have you ever been tested for food allergies? My son gets cramps and diarrhea from watermelon and pineapple. Turns out he is allergic to both of them.
The only test I did was the screening for coeliac disease, which came back negative.
Except for coeliac and lactose intolerance tests, which are well established, other food tests are not offered by medical practitioners in the country where I live (Italy), since there is no scientific evidence supporting them. These tests are sometimes available in pharmacies or in alternative medicine practices.
How did your son find out about
watermelon and pineapple? An allergy, in medical terms, would cause skin rash, swelling, shortness of breath etc. right after ingestion, rather than GI symptoms.