Oh my...It was very scary when my my rhode island red hen, Ros, got into a very similar situation as your muscovy drake. Ros was attacked by my LGD puppy. I found her crouching in terror in front of the LGD, but when I showed up the hen jumped up and pecked the dog. I brought Ros inside. Her skin was completely gone on one of her thighs and on much of her back. Where the thigh joins the body was a large shallow "crater" where not just skin but muscle was gone too. We put Ros in a sink and washed her off with water. By then I had fully expected her to go into shock. But she didn't so we poured Hydrogen Peroxide all over the wound, then coated it with Neo Sporin. To keep flies off the wound, we loosely wrapped gauze around her. She was not interested in food, but she was very thirsty. I had never seen a chicken drink so vigorously before. Much to my surprise the next morning, Ros was alive and alert. We gave her the hydrogen peroxide and neo sporin again and bandaged the wound again. After that we stopped with the Hydrogen Peroxide and New Sporin and instead sprinkled Bird Biotic powder on the wound. Ros did not like that, and looking back on it we don't know if it was a good idea, but Ros survived. We also mixed some Bird Biotic in her water. She was still thirsty. She also got an appetite for pealed peaches straight off our neighbor's tree and would eat nothing else for the next several days. On the third day, I noticed maggots in her wounds. They were big maggots too, and were in every little pocket the wound made. We had heard that some maggots only eat dead flesh, saving the animal, but also heard that only one kind of maggot ate the dead stuff and other types of maggots did much more harm than good. We decided to wait and see what kind of maggots these were, and by mid day, Ros, who had been getting slowly better, was quickly getting worse. The maggots had to go, so we tried flushing out with Hydrogen Peroxide. That didn't help much, so we had to pick the maggots out with tweezers...uggh. Then we made a weak solution of Permethrin (it was not as strong as the dip solution on the bottle. As we recall it was no more than 0.25 ounces of a 10% permethrin concentrate per gallon of water.). We dipped her in the weak solution so that it covered all the wounds. Once all the maggots were out, Ros perked up. She also was hungry for more than peaches and started eating plain yogurt, scratch, egg yolk, and layer. We did not give her much layer because we didn't want her to have to lay eggs and recover at the same time. The first night, when she was attacked, Ros laid a soft shelled egg. After about a week, Ros was limping around her room. She would try to perch on stools and one time hopped into the kitchen. After that she spent most of her day outside in a big cage near the rest of the flock for company. Soon she insisted on perching in a tree in our backyard. She was also tired of peaches, and wanted nothing but bugs. To our amazement, Ros does not limp and the skin is growing back (or at least we think it is growing back because there are feathers covering 100% of the wound). Then, about a month ago, she started laying huge eggs. She now maintains the position of "special chicken" and likes to hang out in the backyard. That is probably a good thing because the other chickens would pick on her if she lived with them.
I hope this helps you and your duck on your road to recovery. Please update us on how he is recovering. Good luck!