The daycare where my daughter went after school and during the summers several years ago kept ringneck doves. One was born with it's legs horribly twisted. He lived just fin in the cage, as he was able to sort of wiggle from place to place. As the school year started in, I think, '96 the staff turned all the doves loose. Including the crippled one.
We were horrified when we found out, but the daycare was next to a park and so the doves at least had water and a shot at some food. The next spring none of them came back. Except the crippled one. It flew into the door when my daughter was there. They were going to toss it out again, so we took it home. I'd never owned a bird in my life, but he stayed with us until just last year, when he passed away. I found that giving him (or his rear) a sponge bath every other day to keep his vent clean and his legs clean was very important. I also made sure that he had a pie plate with very soft cloths in it to lay on, and that he got a little morning sun every day. Made sure he had good, clean water, fresh food (including some fruit), and he seemed to be having a decent enough life. You'll want to keep the little birdy out of drafts, and don't let him stay wet from his bath too long, as he might not be able to shake and preen himself dry. Keep his cage extraordinarily clean, as he's got enough to fight off without having to worry about picking up bacteria from the floor of his own cage.
We took our little guy to the vet when we got him. They said if we'd gotten to him while he was very young they might have been able to straighten out his legs, though he might never really have been able to use them if the nerves were damaged. As it was going to cost over $1,000 at that time, it wasn't doable. None of the rescue agencies would take him, as he was tame. Humane society wouldn't take him, except to euthanize him. So we kept him.
Just keep him clean, help him to not lay on the same side all the time, and give him what love you can. You might rig up a little sling to hang from the top of the cage that lets him sit upright with his little legs dangling down, for him to be in sometimes. It seemed to cheer our guy up. He was a great bird, used to scuffle over to the door of the cage when he saw us and ask to be taken out so he could sit on a lap and watch tv (had to hide the screen from him, though, -he was afraid of it). Hope you have many happy years with your little friend. Our friend passed away about two years ago.