Wayne,
You should start(now is a good time) with a cap gun. Have your wife put the pup's food dish down, and with you in another room (nearby), shoot the cap pistol as the dog is eating (have your wife watch and tell you what the pup did (or didn't do) when you shot the cap gun. If there was no noticeable reaction, move closer the next day. After you can be in fairly close proximity to the pup and shoot the cap gun while he's eating and get no reaction from him, then you can move outside and start using your starter pistol (again from a distance) and slowly move closer when you can tell the gunshot is having no negative effect.