Pain across the wrist during hyperextension described could be from any number of problems. There could be a problem in the bone such as a fracture or bone impaction. Or the pain could be caused by something in the soft tissues. This could be a torn tendon, ligament, or cartilage.
A recent study of athletes with similar symptoms found a new condition causing wrist pain. This is called extensor retinaculum impingement. The retinaculum is a band of fibrous tissue that goes across the wrist. It provides a protective covering over the tendons of the wrist and hand.
But the retinaculum can become thickened from overuse. Repetitive motion causes inflammation of the lining around the tendons. When the wrist is extended fully, the edge of the band puts pressure on the swollen and tender tendons. This pinching causes the pain athletes describe.
A medical exam is needed in order to know for sure what's causing your son's painful symptoms. It could be impingement or something else. The physician will take a history and perform an exam. X-rays or other imaging studies may be needed. Treatment depends on the underlying cause.