Aside from directed training and trying to acclimate him to loud and sudden sounds, I suggest also that you study your own body language.

Turid Rugaas has a wonderful inexpensive book on "calming signals". There are little signs you can use while training, that will help your dog to better understand what you are trying to communicate.

In any case, esp if you have a multi dog household or are involved with watching a lot of dogs and rescues, and doing their training, getting a copy of the book and reading it is a good start in getting past miscommunication with your dogs. I don't agree with every element that Turid interprets but what she has written has been very helpful. You may almost feel like a dog whisperer.