To be honest, a friend has 2 Weimeraners, Mum and son, the son has bitten 3 people now, he is 18 months old, the first 2 he bit were people who knocked on the door, so after that they locked him in the garden when people knocked instead of addressing the situation. The 3rd person he bit was a man cycling through the park (the dog was off lead so no lessons were learnt after the first 2 incidents) This man reported them to the Police and they came round their house to speak to them about the dog. They said as it was the first time he had bitten anybody (

) little did they know, and that the dog was not known to them, they asked them to be more vigilent and make sure it doesn't happen again.
I know exactly how you feel, you are an animal lover and don't want to see anything happen to the dog, BUT, this mans attitude towards you, a woman on her own out walking, is no wonder the dog is the way he is, he seems to think it is acceptable behaviour and just because he is a Doberman should mean nothing at all, I know many a lovely Doberman. He needs a knock on the door to be 'educated'. The Police knocking on my friends door was enough for them to go out and buy a muzzle and for the husband to start serious training with him, they also had him castrated and he has settled down.
That dog could easily bite a child, he obviously has no control over it.