Anterior cruciate ligament (ACL) injuries are commonly reported at all three military academies (West Point, Naval Academy, Air Force Academy). The cadets and midshipmen are all required to join in sports and military activities all year long. No one is exempt.
Everyone must meet a high level of physical fitness to get in any of these schools. And they must keep up this level of fitness throughout their training. Likewise, any cadet suffering an ACL injury is expected to return to the same high level of physical activity after knee surgery.
It may be best not to second guess your son's decision based on this one injury. It could have happened anywhere.