I did have a look and it also feels very smooth, not as if there is anything there. I'm pretty convinced it was a car. He has a balder patch at the thigh area too
A miracle he survived if he had an encounter with a car. I hope he makes full recovery. Might want to fence in your ducks to prevent this from happening again. Oh and Epsom salt is also a laxative so don't let your duck drink the water.
Can he put weight on his leg?