I don't know if this will help, but I had a duckling with his/her legs totally flat to the back. He would pull himself along with his beak.
It was so sad. I called a local breeder and he told me it could be neuro, congenital, or low vitamin.
I put vit & electrolyte in his water and in 3 days he was walking (still with a swagger but walking). I also gave it to all my chicks and ducklings at the time just in case.
Over the next week or so I was able to put him back in with his brooder mates for some supervised socializing (to get him to move about and exercise his legs in his private box he would just sit). Now he walks just fine.
If it was neuro or congenital...I would have had to cull him as sad as it would have made me because he would not have made it. I think it would have broken my heart if the vitamins and electrolytes hadn't worked. I did try to hobble him first, it was a last ditch effort to call the breeder as I hadn't found this board yet.
Good luck. Try the vitamins and if you see some improvement don't forget that you may need some rehab too.
Laney