It looks like he has fractured one of the bones in his fetlock/pastern. His hoof was flopping wildly when he was pulled up and when he was standing waiting for the ambulance. The fact that he will not put any weight on it at all speaks volumes. Horses' hooves simply do not move like that normally. What a shame.
Of course we can't tell for sure just looking at TV, but my initial impression is that the break is in the cannon bone above the ankle.
If he were out of his mind with pain, and wouldn't stand still, he might kill himself like Ruffian did . . . poor mare. I saw that race live and the worst part was when they couldn't get her to hold still.
There's a stud that I read about who has an artifical leg from the stifle down. Took some work to adapt it for him, but he does o.k.