we got him from a breeder's who has been dealing with these type of dogs all her life and we met his entire family from great grandparents to his mother and father, they all seem friendly dogs who barked at first but were pleased to meet us - his mother was a little bit more shy and frightened of us at first but this was her first litter. we were assured of great temperament and good dogs.
Why is the breeder breeding dogs with fear aggression reactivity? And why did you choose to go with a puppy from dogs that are that way? It is genetic and very hard to train out.
The most you can do is keep him under threshold(like the behavoirist suggested/stay outside the fringe of activity~don't totally isolate your pup) and build up the level of confidence, which happens in training. Seeing as you are using a behaviorist, I would stay with the methods they suggest, consistency is important. Hopefully this trainer is very knowledgeable and will help. 6 months is usually the age it starts showing because the puppy is gaining more independence. My own dog, Onyx was the same way at the same age. She got better with maturity and my management, but is a dog that I can't trust out and about. I need to manage her carefully and set her up for success.