That's good news he's doing better. Are you in a climate where he has access to a pool? Or can you put him into the bathtub? He needs to have some therapy and water therapy is the best it takes the pressure off the legs and back but helps build muscle back from not being able to move around much or walk after an accident or injury. Put enough water in so he can float and swim around. He'd probably enjoy a good bath too. Some recover quicker than others it depends on the injury and also the duck. So try not to get discouraged, how is his attitude? Does he seem depressed? You could add a mirror to what you're keeping him in so he sees another duck. They really like the companionship of other ducks.
Did you try making him soup out of his feed? All you need to do and I would start off with a small amount just to see if he is interested just take some feed and add some warm water to it to make it soupy, My flock loves soupy feed I do it for them at the end of the day before they go into their coop for the night. There is also tube feeding which sometimes has to be done and can save a life when they refuse to eat. It looks scary but honestly if I can do anyone can you just need to get the equipment and follow these directions. Is he drinking water? Are you adding any kind of supplement right now to help boost his immune system? https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/
Did you try making him soup out of his feed? All you need to do and I would start off with a small amount just to see if he is interested just take some feed and add some warm water to it to make it soupy, My flock loves soupy feed I do it for them at the end of the day before they go into their coop for the night. There is also tube feeding which sometimes has to be done and can save a life when they refuse to eat. It looks scary but honestly if I can do anyone can you just need to get the equipment and follow these directions. Is he drinking water? Are you adding any kind of supplement right now to help boost his immune system? https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/