Dear Tanya
I am sorry to hear this and I think it's very important that this dog is seen as soon as possible by a qualified companion animal behaviourist who should be able to work out why he is like this. There are many reasons including a fearful character or he might have learned to be fearful and is trying to protect himself. The list is endless.
I would email them back and say that you feel that this would be the next step. Most pet insurance companies cover for this sort of thing as long as the behaviourist is qualified and comes via their vet.
It would also be a good idea to take a look at all your other puppies from this litter, how are they all behaving? Is this one the only one that is showing this behaviour? If they are all behaving like this then it may not be a sensible idea to breed from the parent dogs again. If the other pups are fine then there probably is another reason why this particular pup is behaving this way and this is where a good behaviourist comes in. You might mention to the owners of the pup that the other puppies are fine.
You could ask the new owners how they are training him, has he been attacked by anyone or fearful of loud noise or gun shot. But it is important that they seek professional advice (not just a dog trainer).
One you have all the facts you can decide between you what to do next.
Hope this helps
Best wishes
