That looks just like the problem one of our ducklings had when he hatched. The foot didn't unfold and he walked on the top of the foot with it pointing backward.
The solution is very easy. Cut a small piece of cardboard, like from a cereal box, the shape and size that his foot should be when unfolded, size and shape by looking at his other foot. Just guesstimate, it doesn't need to be exact, but you don't want it too big, either. Then you tape it (we used a bandaid) to the bottom of his foot, being careful to spread his foot gently across it and tape in place.
The first few times trying to walk on the corrective shoe, he'd flip himself or trip all over the place, but it forced him to walk on it the right way and for it to be uncurled, and after three or four days, we removed it. He walked absolutely fine after that.
I'm sorry I don't have pictures of the "shoe" we made for him. I can't remember where I found the info on that.
It's not spraddle leg, though he could have that too, but you wouldn't probably know for sure on that until he has his corrective shoe on. His two hatchmates both had spraddle leg, so I cut a band-aid lengthwise in half, and using the pad in the middle of his legs as the just right distance apart measure, wrapped the tape ends around till they met the pad. I did this just above the "knees" part of the legs. They had trouble walking for the first few hours, though it was much better than when they had spraddle leg, but after a few hours they were really rushing around, and when I took the bandaids off three or four days later they were fine, perfect duckie waddle!
Good luck! He's a cutie!