If he is crazy for the ball or tug use this. When the stranger or dogs gets close enough for him to notice and he starts posturing get his attention pull out the ball and play with him. Take him to public places parks, play with him a lot. Ask for focus, mark then play. Dont have people pet him or any other silliness. Also, correct any negative reaction to other dogs/people and reward focus on you.
Such shy animals are in all circumstances an encumbrance to their owner, who must be ashamed of such a dog, and a disgrace to their race. Under no circumstances whatever must they be used for breeding, however noble and striking they may appear.