If your son is getting weak and sleepy, the main concern should be dehydration
indeed which in some cases can be life threatening. He may suffer from acute gastroenterocolitis due to viral infection. Appendicitis
is also possible especially if the pain has started in the upper middle abdomen and migrated to the right lower abdomen.
I suggest you to visit your doctor right away, have physical examination, blood work and abdominal ultrasound
Hope this will help.