How do I teach my lab to retrieve--and drop?

He likes to go get it and then sit and chew on it. But he won't bring it back. I think I might have done something wrong with him because now he has no interest in retrieving my canvas puppy dummy. What I did was tell him to drop, then pry his mouth open and praise him when he dropped it.

I took a duck wing yesterday and had him retrieve it. He did a great job of going and getting it, and showed a lot of enthusiasm. But he wouldn't bring it back. He'd then sit and chew on it. I'd force it out of his mouth, but after about a dozen throws he finally ate the dumb thing--feathers and all.

How do I teach him to retrieve and undo the bad I've done with him? How do I teach the Drop command.