Your chick has spraddle legs, this is caused when the chicks legs twist out from the hip and will remain in that position unless corrected. To correct it you must brace the legs/hips in the correct position for about a week. Hanging in a sling will not correct the hip position.
Details for proper treatment can be found here:
http://www.the-chicken-chick.com/2012/02/spraddle-leg-in-baby-chicks-what-is-it.html
A sling is used to treat a slipped tendon or hock joint when the chick needs to not bear weight on the leg...completely different conditions and treatments.
Spraddle leg can only be fixed if proper treatment is started in the first few days. Otherwise the condition may become permenant and the chick could die because it cannot get around sufficiently to eat, drink, and take care of itself.