Have you checked to make sure the hock joints aren't dislocated? Pull the leg backwards and if you feel a bump on the side of the hock, massage it back over the joint. You should hear a pop or feel it when its smooth down the hock. It would need wrapping in place for about 2/3 days if it is that.
We had a little one similar to yours and he had a dislocated leg joint, we wrapped him and made him a little chair. Now hes 12 weeks and running around completely fine.
You need to do it as young as possible as the bones grow very fast in chicks and are harder to correct.