The best advice i can give is to start out with a starter pistol 1st. start out at 75 yards away from your buddy, he will have the starter pistol and the dummy.have him throw the dummy high in the air so the dog sees it ,when the dummy is at the highest point have your buddy fire the pitol. then send the dog, its not a big deal if the dog doenst bring it to you right now.we just want the dog to get use to the gun.( we want him thinking guns@ birds guns@ birds!) We want him to associate guns with a good thing,(getting to retrieve something)
when you start to see him less aware of the gun start moving closer but dont rush it ,it will happen. Now that you got this far start all over with a 4-10 doing the same thing,then a 20 gauge and then the 12 gauge.
It sounds like your behind the 8 ball at this point so take it slow and remember to keep it fun for the dog with lots of praise.