This might well be a good time to bring into the family the second dog, and provided it is a puppy or a young dog it will do your boy a lot of good. This kind of aggression is defensive and is a learned behavioral response. Unfortunately fear is mostly influenced by heredity and the environment. I would suggest that you attend a social dog class so that your pet can socialize with other dogs and people under the watchful eye of a professional person. It can be done in a few visits, both of you will enjoy and you will both have better lives.
Most fearful dogs tend to stay quite close to their owner.
This type of dog should be encouraged to have a toy. Encourage the dog to carry this toy as a retrieve article. Play by throwing and gradually increase the distance that you throw the toy. The excitement of the hunting sequence helps negate any fears of being away from you. Try to make yourself less available in general to the dog.