ch 31 and i still don't understand how soyoong hurt his leg, it's the ankle or what?

His a boxer and took a strong blow in the head. You can get paralysed via head injury.

also i think his ex had something to do with it too?

As i said that depends on the gravity of your head injury. Its not necessarily a spine injury, a part of your brain is responsible for your mobility if it gets damage it can paralyse different parts of your body, not necessarily entire but some parts. For example, a person who experienced stroke sometimes can no longer move a leg or two bec it affected his or her brain already.