Have your dog heal beside you at your training grounds and have another person shoot some rounds at maybe some training pigeons while you have control of your dog. Only let him retrieve about 50 % of the fliers, this teaches him that he will not get a bird on every shot. Also work on sending him on retrieves by using his name, not fetch it up or dead bird. This theory will get your dogs attention, and will let him or her know that it is ok to retrieve now. I spend several minutes a week teaching obedience and my dogs are well behaved, but i trained my dog with a whistle, one short blast and he will sit, several long blast and he will return and heal. Patience is the key, but sometimes you will have to get stern with your dog.