There are a few things that can cause this.
Mites- Hold the duck in a clean towel and if he is infested with mites you should be able to see them on the towel. Treat with 7 dust or Ditomaceous Earth.
Nutritional Deficiency- This is usually a case of not enough essential amino acids which can be fixed by adding a higher protein diet and providing whole grains such as whole wheat.
Other birds or cages damaging feathers- If the housing is too small he may be scrapping his feathers against the sides or maybe the other drake is picking at him. Separate him and give him more room.
Dirty bathing (water is too dirty or muddy to properly clean)- Provide a clean source of water for bathing.
Uropygial Gland (Oil Gland) issues- If an infection of the oil gland then antiobiotics may be needed. It could also be a lack of vitamin A, called hypovitaminosis A. Treatment is to provide vitamin A and moist heat to the gland regularly until the gland begins working correctly. If the gland itself is not working then there's not a whole lot that can be done although some have used coconut oil as a replacement. They just stick a bit on the oil gland so when the bird preens he moves the oil around onto the feathers. It won't hurt the bird although it can make waterproofing less effective so they can't fly or stay as buoyant in water. This could lead to drowning if the bird can't get out of the water and hypothermia in colder conditions. Birds with a gland defect should have access to adequate shelter at all times because their feathers don't form as tight a barrier against cold and water.
Genetic feather defect- There is a defect in the genetics that causes feathers to look like they have split ends. It won't hurt the bird although it can make waterproofing less effective so they can't fly or stay as buoyant in water. This could lead to drowning if the bird can't get out of the water and hypothermia in colder conditions. Birds with this defect should have access to adequate shelter at all times because their feathers don't form as tight a barrier against cold and water.