You are doing a Great job.
Yellow belly is a good name. As it describes him well.
Keep him hydrated with water. If he will eat, that's good too.
I'm glad your dog showed you where he was. Please don't be mad at your dog. Most dogs can't help themselves. You will need to keep an extra eye out and keep him safe for now on.
If your duck makes it through, my biggest concern is the oil gland back by the tail area. This glad is very important to ducks. It keeps his feathers and skin oiled up. Ducks must have their Oil Glands, otherwise, they can repel the water.