If he's from the typical backyard breeder bloodlines of the last 30 years, he won't hunt well anyway in all liklihood, so you're wasting your time. Just enjoy him as a pet. If, OTOH, you got him from a quality reputable breeder who kept alive the traditional spaniel instincts for retreiving and not backyard bubba or AKC Annie ( :barf: ), then you've likely got a good upland hunter on your hands. If you're just bound and determined to fight nature and the results of generations of poor breeding (*IF* that is indeed the case), then start off slooooowly & gradually with noises - take it out to the field on a regular basis, and shoot, increasing gradually over a period of months, while the dog is young:
1. Colibris & distance (50-100 yards), then close
2. CBs at distance, then close
3. Shorts at distance, then close
4. regular .22 lrs, at distance, then close
5. small centerfire, like a .22 hornet, .410 bore, or .223, at distance, then close
etc, etc.
If the dog balks & acts scared, either move down the scale of cartridge loudness, and/or increase the distance between you and the dog before firing. A gradual buildup like this teaches the dog to slowly accept noises, until you're at big centerfire or 12 ga, and voila, no problems. Also, while around the house, just clap your hands loudly occasionally for no reason, and/or dry fire some guns, and cycle the action of the guns - if the dog stays near, then reward it with petting & praise each time it doesn't shy away.