Thought I'd throw in my two cents.
I have had a number of bird dogs over the years and prevention is definitely easier than the cure. But, now that you see this you have options. As some have suggested, you can take your dog to a pro. Expensive, but effective.
If you are going to do it yourself. Try this.
Get some good canned dog food. There's some real junk out there. Blue Buffalo brand makes a great canned food, lots of meat and healthy stuff, beef, lamb, venison, salmon. Your dog will love it.
Put the food down. Let him eat it.
Next day, grab a gun and hold it while you prepare the dog food. Don't wave it around or anything, just hold it in the crook of your arm as you prepare his little feast. Talk to him. Act excited about what's happening. Put that yummy chow down.
Next day, same thing, but while he's eating go over stroke his shoulder, tell him what a good dog he is and set that gun down on the floor. Walk away. Watch him. Trust me he'll start ignoring guns.
Next, more canned food. While he's eating, without looking at him bang the counter. Not too loud at first. Just enough to startle him. The food will take his mind off the noise in a couple seconds. Let him eat. Bang it again. Take your time in this. Bang the counter with your open hand for a few days a few times while he's eating this great chow. He will eventually hardly even look up. And that's the key to each step. Your dog will tell when he has accepted this new noise in his world by hardly glancing over. He will be focused on that great chow.
Next get a couple two by fours. Same thing. But this is louder. Ignore the dog while you're doing this. Just look off like nothing is happening. Then bang those two by fours together, progressively louder. All the while feeding him this irresistible food.
Once he's ignoring these noises go to the gun. If you don't have one, buy a 22 blank pistol. You can get one at gundogsonline for about $25 or 30 bucks. Take him for walks by creeks and lakes and rivers where there's lots of things to occupy his senses. You want him cranked on nature. Point the gun away from the dog, behind your back when he's 30-40 yards away. Fire one shot. Keep walking and ignore the dog, unless he runs back to the car.
Fire another shot. Ignore the dog.
Do this for days on end, until the sound of a 22 doesn't bother him. He's a lab, so I know he loves to play fetch. Get him on stay, whoa, hold, whatever you use to make him sit still, or have someone else hold on to him. Take his favorite retrieving dummy throw it as far as you can and pop that 22 as you throw it. Now he will associate the sound of that little pop with the great joy of retreiving. Keep this up for many days. Move up to a .410 and start all over at the creeks and rivers and lakes. Stay away from him when you fire. Not too close. Watch him closely, he'll tell you what's up.
Take the dummy and throw it in the water. Fire off the .410. See what he does. If he looks nervous, back off.
Next step....birds. Here is where exteme caution needs to be used. You do not want any, and I mean any, negative associations with birds. So back to the 22. Get some pigeons, chuckar, whatever. Get him into a couple birds and just let them fly off. Get him all cranked up, bird crazy, birds, birds, birds!!! Working a field, searching for birds. Get him on point. Next, pop the 22 as you flush a bird.
DON'T WATCH THE BIRD!!! That is the mistake of the weekend trainer. We like to watch birds fly, and then shoot at them. This is all about the dog. Watch him as you flush the bird, then pop that 22 in the general direction of the bird flying off. He is going to be focused on that bird. Do this a lot. Get him used to the sequence...smell bird...point bird....you flush bird...bang!
Then move up to the .410 and actually kill a few. He'll be ignoring guns, focused on birds and retreiving before you know it.
I'm no expert, but this has worked on every dog I've owned. However, if at any time he is not adjusting to the bang at any level, you are either going to fast, or you need to seek professional help for your dog.
A professional trainer will cost you $500-1,000 bucks to fix this. Trying this method, maybe $50 and some gas, along with some great time with your dog.
And I can't emphasize enough that you need to take your time. This is the hunting companion you want to spend the next decade or more with. He needs to trust you in this environment. Taking your time with this will build that trust, and a great bond.
Good luck, and keep us posted.
Thanks,