I am not a proffesional but may I suggest driving him to a safe location like a park or school with a enclosure that you will be virtually alone and safe. Try to engage him in a game of fetch or sit down with a book and just let him smell the roses.
I have actually heard of people sedating their dogs when events like this happen but that hopefully you wont have to go that far. Its strange how some dogs are scared of random things like vaccums, balloons, and trash bags, although yours is on a much more scary level. I do know that this is a very real fear (as you have found out) and while we are not supposed to encourage this behavior by trying to comfort them verbally I'm hoping anotehr member will have some advice for you on how to make him feel more comfertable. Good luck, oh and here in my town we dont allow fireowrks, maybe you should come her for 4th of July, ha ha!