I keep two pet ducks inside all of the time... here's some advice/tips for keeping your duck in for a night or two...
You can keep your duck in a cardboard box or in a rubbermaid plastic container (no lid obviously), for a night or two. you can line the bottom with pine shavings, horse stall pellet bedding, or even newspaper if you don't have any bedding. I've used puppy training pads as well. that will help absorb some poop/liquid.
You don't have to feed him throughout the night, but if you decide to leave food out for him, you have to make sure he has access to water.
Ducks can choke on dry food and need water to swallow properly.
While in a box/tote etc you can cover the top with a light blanket. that seems to help with the duck being nervous about being somewhere different. The duck can also stay in the tub for the night, but it's better to put some newspaper or something down so the tub bottom doesn't get all slippery when he poops... last thing your ducky needs is a leg injury.
If your duck is people friendly and your grandma will allow it, bring him in his box into your room or a living room while you are awake and at home... that way he can have some company and some attention. You can also watch him better to see if he picks at those feathers himself. If he's not picking at himself, that will be a good indicator that it's rats chewing on him. Ducks will eat their own feathers in cases of mal nutrition.
He def needs to eat more than whole corn. I give my ducks corn everyday but as a treat. He can be transitioned to a crumble diet by making a mash as others have said. You can try adding less water each time too. Also offer him whole corn with the feed your grandma got in a dish. Sometimes ducks don't like to eat in the presence of people... mine sometimes wait for me to leave them alone before eating. I don't know why, but my duck victor won't eat his treats in his bath unless I close the shower curtain in the tub, lol.
You can also give your duck a nice warm clean bath inside that he will appreciate. The water can be just barely warm to the touch, not hot. IT will help sooth those poor feathers. If you have access to a pet store or a
walmart, you can get him floating Koi food, which is a pellet meant for fish. It comes in a orange can or a green bag at
walmart. It's 30% protein and makes a good treat while in the tub. it gets soft in the water and might intice him to eat more.
Also, try feeding him a variety of other treats, like peas, lettuce, tomatoes etc... You can find a huge list of treats you can give your duck by clicking the link in my signature...
Your little guy really needs more nutritious food, and a friend. If you can get another duck, please do so. Ducks do not do well by themselves at all. My ducks will cry for each other if i seperate them. If you can not get another duck, you may want to seriously think about giving your duck to someone who has ducks. He most likely needs medicine because he probably has an infection in those bloody feathers. Taking him to a vet or to that rescue is important for the life of your duck. He could die from an infection. Paying for an animal to receive care is all part of being a pet owner. I had to pay over 400$ last winter when my duck had a foot infection and needed xrays and surgery. My ducks are a little over a year old and i have spent a total of 800$ on vet bills so far. considering ducks are cheap to feed and keep clean, and they don't need expensive toys or shots, the biggest cost of owning ducks is taking them to the vet when they need it. If you can not afford to get your duck healthy, wouldn't it be better to give him to someone who can, rather then him suffering and potentially dying? Just give those situations some serious thought...