I had a dog, a rescue, that we were sure was abused. He was great with us for the first six months, then put me in the ER twice with face bites. I was totally lost. Then my vet suggested an animal behaviorist.
This man was a miracle worker. He explained to me why the dog was doing what it was doing and how to correct it. Forget all the things you "think" will work. Dogs do not have logic. We want to think logically, like exposing them to environments, etc. Dogs have no clue about this. It could be you are exposing him to things that will only make him more fearful.
I think the trainer you went to is not the right fit. My contact told me I could save my dog's life, but it required a commitment from me. . . not the dog and not the trainer. Never once did he say my dog's case was hopeless and it would never work. He did all his consultation over the phone, gathering info about my dog and me and what I was doing, etc. Two sessions totaling $120 was what it cost me to learn how to redirect my dog's behavior. A bargain.
It's been so many years ago, I don't know if that behaviorist is still around. He was a veterinarian who later exclusively did animal behavior.
I know a lot of people don't like Cesar Milan, but a lot of the techniques my guy used were similar and worked just as well as Cesar's and this was years before Milan came on the scene. When my sister was having great difficulty with her one dog, she downloaded episodes that pertained to the behaviors she wanted to change and worked those with her dog. It worked for her.
I would try that first. I truly believe the best solution is for the owners to work with their dog, instead of a stranger. And I wouldn't want anyone beating my dog into submission. Incidentally, my guy asked me if I had hit my dog. I have to confess when he ripped my lip apart, I did hit him. Not hard and only once. The vet said. . . "Well, the next time, he will bite HARDER because in his mind he didn't get his point across." Most fearful dogs do not trust their owner to take care of them, so they become aggressive to compensate.
I would run from the trainer you first contacted. Try some of Cesar's video's. Then ask your vet for recommendations. The right behaviorist is the one who has YOU make the behavior modifications.
I agree with Dr. Doug. While my dog didn't need any meds, it was important to deal with the vet professional. Also, as I am sure, the vet has ruled out any medical problems.
Good luck and thanks for not giving up on your dog.
Dale