If you want him to retrieve as a job (hunting, obedience, etc.) then you have to treat it less like a game and more like an exercise, much like you have taught sit or leave it. You can still have fun and praise, but as you know, there's a difference between playing around and training.
Does he lay there and chew it? Ignore it? Run away with it? And, what do you do when he doesn't bring it back? Knowing this will help.