If you decide to make sling make it so the gosling feet touch the floor enough that it can push up enough to be exercising those leg muscles. You don't want it to be able to push up enough it comes out of the sling just enough that it can use the legs to exercise . Other than that and taking the gosling to an avian vet for an evaluation I am not sure what else you can do.
@DuckyDonna
@DuckyDonna