Put him down? I am sure there are more life friendly ways to handle this situation. If it is just an infection you may be able to lance it? Wash it everyday, keep something over it maybe? I would hate to see someone put an animal down for something that might not be to hard to fix. Or i have an idea. if it is just a mass of infection and you can lance it, you could keep it clean and get a diaper for waterfowl to keep it from getting worse. Use some gentle form of antiseptic like peroxide maybe? i am not sure what i is safe for ducks but i do know that it wouldn't burn to much. Then rinse it with luke warm water. Its just a suggestion if you can get it cleaned and figured out that is another option. I dont suggest putting an animal down unless there is something HORRIBLY wrong and it effects the animals life (like a broken neck or back or something that severe).