The problem can be corrected, but it will probably take a long time to get him past the gun. I agree that getting the dog on alot of birds first will help. The dog has to make an association between the birds and gunfire. I would start with just getting him excited on birds or a training dummy if birds are not an option. Gradually start to introduce a cap gun or training pistol from a distance. Patience is very important, since this will take weeks to maybe a month or more depending on how often you can work with the dog. The dog will let you know if he is getting acustom to the gunfire by staying excited on the birds or a game of retrieving a dummy. If the excitement stays with the dog, then you can more the gunfire closer to the dog and more often. Try using a 410 or 20 gauge shotgun first if you get him past the training pistol.