true gun shyness is very difficult if not impossable to break. The best situation is to introduce the gun correctly to begin with. Letting the puppy eat while you shot a blank pistol in the distance and each day moving closer and closer untill you are very near the pup as it is eating and the gun fire doesn't affect him. There are other method and this is just one I have heard over and over again. you may want to try this but if the dog is already afraid of the gun fire it may take quite a bit to bring him around provided you are ever able to fix the problem.
Good luck and let us know how you turn out.