Horrible thing to watch. I have to wonder if there was something already wrong with his leg. It was not an unusual looking landing.
My middle daughter broke her ankle in gymnastics - just on a landing, a similar situation. We discovered she had a bone lesion - on the x-ray it looked like something had just taken a bite out of the bone. That weakened that spot, and when she landed it broke. Fortunately, hers was not a compound fracture. They wound up doing a bone graft, and its better than new now, but it was a tough thing for her to go through.
Hopefully he is able to recover fully, but I would not be surprised if basketball was over for him. Very sad.