We met with the surgeon this morning. She confirmed the initial diagnosis of a right cruciate ligament tear, and said it's likely completely detached from the bone. She also suspects additional damage to the opposing ligament and possibly to his meniscus.
Parker will have surgery next Tuesday, which unfortunately was the earliest he could get in. He'll have a right tibial leveling osteotomy, in which she will carve out a piece of bone and turn it so the damaged ligament no longer supports his weight.
After surgery, Parker will have a few really miserable weeks. For the first two weeks, he'll be confined to a bathroom, forced to wear a Cone of Shame, and only allowed short potty breaks. After his sutures come out, he'll still have restrictions on his activity for up to 6 more weeks. But by the end of May, he should be able to go on long walks again.
When the surgeon goes into his right leg to repair the damage, she'll also evaluate whether he injured the knee through an athletic over-reach or whether it resulted from deterioration of the ligament. In the latter case, there is a 50% chance of his left knee blowing out within a year. So I'm hoping that this was just the result of over-exuberant stair-climbing on his part, and not the result of age.
Updates as conditions warrant.