Your son's injury is called a "high-energy injury" because of the force of the impact. His injury was caused by coming down on the heel with his full body weight at just the right speed and angle.
Pain long after the injury has healed is common in cases like this. Sometimes unseen damage to the bone occurs. This can affect how the bone moves with the other bones and joints in the ankle. Ask your physician to re-examine your son if this has not been done yet.