What Causes Weird Feeling In Abdomen While Doing Exercises?

MY 15 YEAR OLD GRANDSON IS COMPLAINING ABOUT A STRANGE FEELING IN HIS STOMACH ON BOTH SIDES OF HIS BELLY BUTTON BUT ONLY AFTER EXERCISE THAT INVOLVES RUNNING AND/OR SIT UPS. HE FEELS A "SQUISHY" PLACE IN HIS BELLY THAT STOPS HIM FROM FURTHER EXERCISE. HE FEELS LIKE THERE IS A SWELLING BUT AFTER REST IT SEEMS TO GO AWAY. HE THOUGHT HE HAD AN ENLARGED SPLEEN BUT I DON'T THINK HE WOULD BE FEELING NORMAL OTHERWISE. ANY THOUGHTS.