My I suggest some thing you need to go back a step or two and get reset. Then when you want to introduce gunfire you will need a second person. Let Jake chase and retieve bumpers having fun. Have the second person atlest a hundred yards farther is better behind you and Jake. As Jake is running too the bumper have your second person fire one shot. If Jake is ok with that do it again after a few times let the second move about twenty yards closer and repete. If Jake is ok with it again move twenty yards closer.
My last lab I had years ago and at the time, had trouble finding a dependable partner to assist with the training. I went to a public skeet range and threw bumpers for my dog across the street from it in a nearby field. I worked my dog closer each day or two and he never had an issue his entire life. Just a though/option