You need to get him to a waterfow/bird veterinarian. He may have an infection, parasites or something else going on that keeps him from keeping his feathers tidy. If he cannot keep dry or warm, he can get a stress related illness and even drown because of the lack of water repellency. Has there been any changes to his habitat, i.e. domesticated or wild predators, new plants or other additions or deletions from the habitat? Has there been any changes to other swans in the habitat, i.e., nesting or mating fights, etc.? Any of these could be a cause, but he needs to be examined for infections, parasites, fungus or other problems to rule out any systemic issues. The Regal Swan