I’m wondering if he’s injured at the hip. He clearly can paddle, but seems like he can’t bear weight. Are you familiar with chicken slings? I wouldn’t leave him in one unattended, but wonder if he might like short periods in one so he can stay upright.
Oh, that’s big, I think! Like one of his air sacs was punctured? Was he examined by a vet when he was x-rayed? If not, can you look over every inch as suggested by @casportpony?
It looks to me like his left leg is injured. Does anyone know if ducks can use slings like chickens can? And if it’s safe for him, can you post a video of him in water?