PCL's are not as "necessary" for joint movement than, say an ACL is. With the anterior cruciate ligament in place, the joint is stable enough for side to side movement...and the PCL is simply a buffer, not a necessity. As an ACL reconstruction patient, I'd also recommend your son NOT have reconstructive surgery. Although competitive sports might be important for him now, he may not be as athletic later in life...and an invasive surgery may harm more than help. I am athletic but have also developed arthritis...and find that any running aggravates and already damaged joint.

Let me know if you've got any other questions...glad to help.