Poultry vitamins in the water that contain trace minerals would be a good thing to do in case of deficiency. Below is a link for a good one--dose is 1/4 tsp per 5 gallons water. Also making a splint or shoe may be helpful. Here are some links to help you find out what leg problem it has--slipped tendon, twisted tibia, or a vitamin/mineral deficiency:
http://www.jefferspet.com/avian-super-pack/p/AB-P1/
https://sites.google.com/a/poultrypedia.com/poultrypedia/poultry-podiatry
http://nhjy.hzau.edu.cn/kech/synkx/dong/2bao/Bone disorders in poultry.pdf
http://www.thepoultrysite.com/diseaseinfo/30/chondrodystrophy-slipped-tendon-or-perosis
http://www.thepoultrysite.com/publications/6/diseases-of-poultry/220/slipped-tendon-perosis
http://www.jefferspet.com/avian-super-pack/p/AB-P1/
https://sites.google.com/a/poultrypedia.com/poultrypedia/poultry-podiatry
http://nhjy.hzau.edu.cn/kech/synkx/dong/2bao/Bone disorders in poultry.pdf
http://www.thepoultrysite.com/diseaseinfo/30/chondrodystrophy-slipped-tendon-or-perosis
http://www.thepoultrysite.com/publications/6/diseases-of-poultry/220/slipped-tendon-perosis