There's no word about how his knee/leg is but he was lucky to be surrounded by people who were able to catch him before he really fell in :(
He was walking from one part of the stage to another and fell into a random hole in the stage. It's unclear why the hole was there but it was definitely not suppoused to be :( One leg slipped in and he completely lost balance. He was going to fall further in when Seventeen and Pristin members grabbed him!
Let's hope he wasnt too seriously hurt!!