My duck pepper had very similar case just like yours. Up until about a yr ago he could walk despite having his issues. He slowly started walking less and less and then stopped eventually. He would use his wings to add crutches until his wings seized up as well. All I could do is make him comfortable in my home since he could no longer be outside. Warm baths and then just eventually quick dips because he did not like being in water to long is what I did for exercise and cleanliness. His feathers even lost their coating and he was not molting any more either. Also losing patches of feathers. Sadly pepper recently passed on Dec 7 at 3yrs old. He lasted a little over a yr in that condition. Which I thought was a miracle not knowing what exactly caused that. I did everything between niacin and vet visit but his condition never improved. When I came to that conclusion all I could do was make him comfortable with a life inside which I did. Worst was I did have to separate him from his brother Jack but there was no way pepper would have survived outside for very long. I got him a bed, brought his food and water closer and when he wanted to move spots I would just assist him. All i can really tie to his condition was we had a brief cold spell and after that he was never really the same. Here are some pics of him from a duckling to about a couple months before he passed. He is the tall duck with the white patch chest.
My advice is just keep making him comfortable as best you can. A wheel chair sounds great and I wish you all the best for your little guy.