This behaviourist put most of it down to the way he was brought up with his mum and the breeder. We spoke to my partner's parents and they explained how the breeder's place looked and how he was treated prior to picking him up. This breeder definitely didn't bring up the pups properly at all, they were caged separately outside and not socialised or handled. She thinks his mental state has been slowly declining his entire life without anyone noticing, or he has always been somewhat like this and no one has previously picked up on his stress signals.
She sat and just watched him for about 2 hours, she said not one behaviour of his was normal and that he is constantly 'checked out', lights are on and no ones home. Explains why he is very unpredictable as he basically has no control over himself.
So we are keeping him in a safe spot, alone and making his world very small to bring his levels back to normal with the help of medication. The behavourist said because he is a very emotional and fragile dog, we will always need to be careful to not stress him out or we will continue going in circles. Basically, we now need to make sure hes always put in a different area where he can't hear/see anything we do, when we bring in shopping, move furniture, play with our other dog, etc as he is so fearful of anything and everything.
We will also be getting another lot of medical tests done just to make sure he will be ok on this new medication. So far we've spent about 1.5k on this dog just with these behavourists and tests! Hopefully, this medication is the answer we've been searching for.