For Black Labs, first train him to stay. Then train him to retreive a ball from stay. ( I used bright red or orange rubber balls, so I could find them in the water). Then make him stay, throw the ball into the water, and shoot at or near the ball with shot gun. He will likely bolt after the ball, but thats OK.
This technique has worked for years for me with both setters and retrievers. You have to train him to stay through the shot if you hunt from a boat. I sort of let mine bolt, it builds their initiative.
Then you can graduate to multiple balls, 3 or 4.
My dogs would never stop