If that duckling is that young I think you would have much better success if you hand raise it.

When I was a kid on the farm we always took the ducklings and baby geese away from their mothers and brought them in the house where we kept them in a small cardboard box with a small heat lamp to keep them warm. The baby ducks can eat the same dry mash that you feed the older ducks. We just put a little pan with the mash and a pan of water in the box.

When they are about three weeks old you can put them out in a small enclosure in the yard where they can eat grass as well as the mash. At about 5 weeks old you can turn them loose and they will mingle right in with the older ducks,

We have raised four ducks by hand and are actually currently raising three more ducks that are three weeks old. The problem is that this little guy is wild, he is really fast, jumps really high and bites viciously ( not that it really hurts at the moment). I was hoping to have one of the females raise him since he seems to take to them and not to us. It took us forever to get him to eat or drink and even now we need to hand feed him, literally. He seems to require more attention that store bought ducks and since we both work that doesn't work out. All of the big ducks bit at him except for Harley but he was freezing cold and trying to snuggle up to Harley who kept walking away. We tried placing him with the three week old ones and they bite at him really hard. I plan to release this guy as soon as he is old enough if it is possible. But if I raise him by hand I couldn't do that.

Where do you live? Goggle "wildlife rehabilitation" with your state and get the baby to a rehabber.