I agree with the other suggestions of a behaviorist. It sounds like it's gotten to the point that you really need to do something quick. Let's face it, we live in a sue happy society and you don't want anything to happen. This may be the very reason the dog ended up in the shelter to begin with. I would definately contact the shelter also and see if they have more information on why he was given up. The unpredictability of his aggressiveness is what worries me.