Oh well, you will just have to play fetch every time you see a duck, LOL!Yeah, well, good thing I didn't alert the media. Seems it was a one-time only deal. He's back to the chase and drop method. Sigh.
Have you tried turning around and walking or running away as he is coming to you. Maybe clapping calling him or making a whistle noise to encourage him to come farther as he is coming back. It may just distract him and he might just drop it, but it is worth a try.
Do you have a fenced in yard? You could try playing inside then immediately go out side and try playing.
Maybe work on trading the ball for a treat and then increase distance.