Guns = Fun
Is the simple equation even for the nervous pupil.
Take him for a run near a clay shoot, hunt him in the cover, play ball, all the fun things you would do usually.
If you want to work him start with a muffled pistol shot on a retrieve whilst the dog is stopped say 10yds away, pick the retrieve and lots of fun. Don’t overdo it to start with
My Blue dog was a bit timid on the gun to start with, I now shoot over him.