is very nonspecific symptom. It can be caused by organic diseases or by functional diseases. In his case it is obvious functional disorder
because diagnostic tests that you have mentioned would rule out any organic disorder. At this age pain can be caused by food allergies, IBD
or psychogenic. Also any infection of respiratory or urinary tract
can lead to abdominal pain.
I suggest to wait and see are symptoms gonna persist and getting worse. If yes then complete gastroent. workup needs to be done. Wish you good health