In a child of this age, very often appendicitis may start off as pain around belly button and then move down to the right lower abdomen. It is usually accompanied by fever, sometimes nausea or vomiting. In the case of your son it may be appendicitis or something like indigestion or intestinal worms or infection.
Next Steps
Please visit a general surgeon, as clinical evaluation and examination is necessary. You may also need to undergo some tests like ultrasound abdomen.