Beautiful looking pup.
I agree with all of the advice here. You should try to slowly acclimate your puppy to increasing loud noises. Also, if your puppy is highly motivated by a toy or food you can try distracting him. Goal is to have him so focused on a kong or a treat that he isn't even paying attention to the noise.
However, you need to understand that noise sensitivity is largely genetic. You can try testing your puppy with a coffee can filled with gravel, marbles, coins. Get his focus and then drop the can or shake the can in front of him. Most puppies will startle. What you want to see is that he quickly gets over this. Try rolling the can on the ground. Ideally your puppy will soon be pouncing on the can. If he continues to be skittish, you may have an uphill battle getting him used to gunfire.