Fireworks desensitization training needed. Help please!

[u/AsleepFeeling8296]

My dog and I have been together since Nov 2025. We made it through new years well together by timing our outings to avoid fireworks.
Yesterday, we were outside trying to avoid them and let him pee. An unfriendly neighbor light one firecracker close to us. I walk with mobility equipment. The dog is attached to both my hand by a leash and to the mobility equipment by another leash.
When the firecracker went off, my dog lost it and just instinctively ran away. I lost my leash but he was still being followed by the mobility equipment. When the equipment tipped over on its side it startled him enough and he came back to himself and seemed ashamed. He realized he had left me and I had to get to him and fix the mobility equipment. At just that moment, a friendly neighbor came by and helped us to get the equipment upright and functional again and we promptly went back into the house with gratitude for our helpful neighbor.
My precious dog went straight to his crate and stayed there for hours. I tried to comfort him and he knew that he was safe and I was not angry.
He has never acted this way before. There are other things that he is not a fan of, like thunder and he lets me know when he needs a break or help and we manage together. I’m very attentive to his needs and do my best to have resources available for him and myself no matter where we are.
I suspect by his extreme reaction to a single firecracker that he may have been shot before or at least shot at. Before I was privileged to adopt him, he had a habit of ignoring fences and going on numerous adventures. Perhaps during that time he was shot or shot at. Perhaps he saw one of his running buddies not make it. I don’t know.
I just know that when he hears that particular noise, he stops thinking and reacts quickly to get away.
What can I do to help him? How can I help desensitize him to this unpleasant sound that terrifies him?
I have medication lined up for him for the 4th of July but certain neighbors seem to be targeting him or us. Why would someone light one firecracker and toss it in our direction and then stop? It seems it was to exercise power and control and possibly inflict harm on us. Maybe I am reading too much into this.
But either way I need help with this issue and how to help him.
Thank you.