Aggressive dogs are dangerous and their owner should seek professional support to rehabilitate them back to a trustworthy level.  First identify why and when your dog is aggressive. Estimate how much he can improve and formulate a treatment plan.  Discuss this program with your veterinarian and take is seriously. Gary Landsberg, a Veterinary Behaviorist says that one way to help your dog is through desensitization or counter conditioning.  This means that when the pet is put in a situation where it might be fearful or aggressive, they will make a good thing happen. So talk to an animal behaviorist or your veterinarian to learn more.  They may suggest using natural therapies or pet medication in the treatment plan. It is very difficult to control your dog if he is aggressive. Every time someone visits you, you have to hurry and lock him up. This is not fair to the animal or to you so get help on aggression as soon as possible.