I'm sorry to hear the whole leg is curled up, that I can't help you with. I had a duckling that I got from a friend with a really crooked foot and one that was less crooked. I believe she was stressed in the hatch too. I was able to straighten one foot, but not the other. I didn't realize the duckling had curled feet for the first couple of days. I was too slow (started treating at age 3-4 days) or I might have been able to save both her feet. I tried several things, most of them were too heavy, she couldn't move. I ended up simply using making tape carefully pressed to straighten her feet, wrapped around the foot. As she got stronger, I continued to try to straighten the worse foot, her whole leg turned and I couldn't find anything strong enough to put pressure on it that wasn't too heavy. Putting the spoon part of a small plastic spoon under her foot and taping that on worked really well on the good foot though, that turned out fine.
If you want to straighten both legs, you will probably have to do things that make your duckling uncomfortable. You will want to start today and be diligent about it. For the leg, I can throw out ideas, but I'm hoping someone else has some too. I would try to find a way to splint the leg with something stiff, lightweight and that won't poke or cause abrasions. One of the things I tried was the little wooden spoon you get with ice cream cups - those were a great size. I cut one because it was too long and didn't think to round the edge and it caused a small owie before I realized that was a problem. I managed to get the leg fairly straight, but the foot still turned under and she ended up walking with her weight on the outside of her ankle and her foot turned in. This would have been fine in a small breed duck, but she was a Silver Appleyard and, being a large breed (about 9 lbs at maturity), they are prone to leg injuries and foot problems to start with. She got along decently, but I knew in my heart she was going to have a really hard time with it in bad weather (slippery footing) and she was even more slow and clumsy than a normal duck. Since mine are not house pets (though still my pets in my heart - all 25 of them have names!), when we butchered the excess Saxony babies, I did her too. That was hard, but I had time to get some distance emotionally and I knew it was best for her in the long run. However, if she had been a Khaki Campbell, or a Welsh Harlequin or any other smaller duck, I would have kept her, I think she would have been fine.
Sorry I don't have better news, but do some research online too... and tape those feet with a stiff but lightweight tape, so that they are spread out. If you can get him walking on the bottoms of his feet, he'll be fine. I never could find a splint that really worked for my little girl, but I think if I'd been able to tape it from day one, it might have gone the other way. I wish I could be more help!