I have a GSD and have kept them for years, they are my chosen breed.
They are wonderful loyal, protective, soppy with their family and brilliantly trainable dogs that can be highly dependable and steadfast in situations that would make other dogs nervous
But they take a good owner and he is clearly not one.
Stock fencing has no hope of keeping a GSD in - mine can jump our 7 foot fence and wall combo, he just chooses not to because we trained him not to. To be honest no fence should be relied on to keep them in.
As for hedges - no I don't think even thick brambles will work. They are stubborn and determined when they want something and will find their way through
This man has turned these beautiful dogs into a danger - even he isn't safe from them. German shepherds are the last dog I would expect to bite their owner - they are insanely loyal to a point of it being a weakness - if they are turning on their owner they don't see him as worthy of that loyalty and for a dog that is usually all about their family that is saying something
You need to report these dogs to the police, to the council, to the RSPCA and repeatedly until something is done. These puppies deserve a better home and life, they are not living up to their potential and will become more and more of a liability
As for him liking them aggressive.... if he thinks dogs like that will protect a property he is wrong. A well trained protection german shepherd is reliable and safe around the public, they are steadfast and unphased, they only protect when called upon to do so by command. These puppies are running on adrenaline, fear and instinct. They are no use to anyone as protection, their owner is more likely to get hurt by them in a guarding situation.