Phobias like you mentioned, are pretty common. If you catch it early enough and work with a good trainer, you can get him over that. The longer you wait, the worse it will get....honestly...you can depend on that. The reason this happens, normaly, is because they are not socialized very well. Also contributing to that, is getting older. As dogs age, phobias only get worse, almost never better.
Small, seemingly benign noises become something to fear.
We put to sleep my standard poodle last year at the age of 14. She was my constant companion until I retired. She was the most socialized dog I ever knew. Shows, obedience trials, client homs...I even could take her to concerts and walk right up to a band....she wouldn't flinch a muscle.
A few years after I retired, she began, ever so slowly, to be afraid of a ticking kitchen timer I had and used.
By the time we put her to sleep last Sept., I had long gotten rid of that timer because she reverted into a shivering mass of mush anytime she heard that damn thing.
Her son Rio is now soon to be 13.
If we get out a fly swatter, or make any kind of slapping sound, he tucks his tail and heads for our bedroom where his bed is. He is afraid of lightning and thunder and fireworks too.
I haven't started any formal desensitation training tho. We rarely get thunderstorms, and the few times a year firecrackers are lit off, we just deal with it. If we give him a tennis ball (something he LOVES)...he will play with that all by himself, and it lessens his anxiety.
A little tip....you mentioned he looks big and scary, especially when he barks.
If you practice his obedience skills, you never have to do anything like guard/attack training if you want them to be a deterrent to theives. A well trained dog like that, few will mess with.
Thieves, especially ametour ones, have no clue if they are guard trained when they listen so well to other commands. So, when you answer the door to a stranger with your dog, looking mean and listening to your commands like sit and down....any would-be thief will look elsewhere.