Not all dogs are cut out for personal protection work. Some dogs are afraid of their shadows and always will be--if they have been hit or yelled at they have very low self-confidence and are unlikely to defend.
You can increase your dog's confidence in himself by taking him to obedience classes. If he does okay, you can enroll him in more advanced classes so that he will learn to be a guard dog.
A dog not suited to be a personal protection dog can still be an excellent guard dog. All he needs to do in most cases is bark, and let the potential invader know that he is willing to defend his territory.