Get a cork screw stake and tie him to it. It is best to simulate the situation and discipline him when he breaks. If you have not worked on it with him, you can't expect him not to break.
Most dogs will break at some point. Just train for it to minimize it.
I used to bring a leash and attatch it to the ecollar on my dog, then lide the loop through my wader belt. This kept the dog at my side.
#1 most important thing, if the dog is young, make sure you set him up for success by being able to see the bird fall, if not every retrieve is blind, and can be tough for younger dog. A seasoned dog will just usually know where the ducks fall, usually in the decoys. My dog will be sleeping behind the blind half the time and when the gun goes off he is pretty used to seeing the orange feet kicking in the pocket, so I don't worry about him seeing the bird fall....