All the above info is good to prevent gunshyness but you already have the problem, if it were me I would get some pigeons , tether them them with about 20 ' string, cord, whatever.,tied to a plastic bottle so it won't fly very far. let the dog point the pigeon , probably on a check cord to start with, flush the pigeon while he is chasing the pigeon fire a cap buster or 22 . if he shows shyness pay him no mind keep his interest on the bird.He must associate the noise with something good, he is interested in. This works very well but should have been started at first hint of the problem. I have seen very few older dogs cured from this problem sorry to say but good luck. Hope this helps.
