Duckling hatched with bent leg
Looks like it could benefit from at shoe to hold that foot in the correct position for a few days. You'll want to cut a little piece of cardboard or thin plastic in the shape/size of his foot, then tape his foot (fully spread out) onto that piece in the right position. Keep this on for a few days and then remove and see how it looks. If it still isn't quite right, tape it again for another few days. This often fixes these issues. The duckling will struggle and have a hard time getting around at first, but he should adapt quickly to having the boot on. Just make sure he can get safely to foot and water without falling into and getting stuck in any water.
Depending on how straight you can get it with just the shoe, he may also need to be hobbled. That's just putting a thin strip of tape between the legs, attaching the bad leg to the good one to keep it in the right position and angle. Hard to explain, but there are pictures on the Poultry Podiatry site. It is great for reading about foot/leg issues in all types of poultry and how to fix them: