Bad Legs. How To Fix?
Try a vitamin/mineral supplement in water and administer it to him via eyedropper several times throughout the day. Hen was likely deficient in certain vit/mineral.
Retarded growth and severe leg weakness are the first signs noted when chicks are deficient in vitamin D3. Beaks and claws become soft and pliable. Chicks may have trouble walking and will take a few steps before squatting on their hocks. While resting, they often sway from side to side, suggesting loss of equilibrium. Feathering is usually poor, and an abnormal banding of feathers may be seen in colored breeds. With chronic vitamin D3 deficiency, marked skeletal disorders are noted
It could be D3 or Vit E or some other mineral. I would just get some supplement broad in scope to add to his water or all of them. I wouldn't overdo it on the other chicks though as too much could be bad. You might make up some special for this one for the next week or so.
The sitting on hocks that the chick displays, I've read about before and am sure it is related to poor nutrition in the hen's diet. So I think the addition of vit/min will help this chick. I will see if I can find where I read it.