A B vitamin complex, possibly some Vitamin E... DVL makes a product called Poul-vite if you can find it in your area, otherwise there's a bunch of different things out there. Check with your local feed stores typically they have some kind of poultry vitamin mix, with or without electrolytes.
@Eggcessive @Wyorp Rock any suggestions?
I would try vitamins - at the very least it wouldn't hurt. If it's a slipped tendon or an issue with hatching that might be harder to fix....
Could possibly be a vitamin deficiency or a slipped tendon... Are you supplementing vitamins? Did he have this issue immediately after hatching or did it develop? What is your brooder setup like?