I was hoping to get a response back to my request for more information. Generally in cases like this, obedience training helps a dog learn to trust you to make the decisions on when someone is a threat to you and when they are not. So the first thing I would suggest is obedience training. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Once he is trained, you need to work on the aggression itself, using a helper and muzzle. The muzzle will help avoid accidental bites while training. Use a basket style muzzle as they allow the dog to eat drink and breath normally. It will be helpful if you can find someone to help you once you have your dog listening to commands consistently. What you will do is have your dog on the leash. You will have your helper off in the distance. Your helper will gradually move a bit closer to you preferably walking past your position in the distance. As long as your dog ignores them, you can give your dog praise and a treat. The second you see him fixate on the helper or show any other sign of aggression (hair standing up, etc.) give your dog a correction by giving a short tug and a firm low toned "NO". It shouldn't take your dog long to realize you will not tolerate the aggression and that if he ignores the other dog, he gets treats. Once this happens you can repeat the training moving the helper closer until he is no longer acting aggressive. You will need to practice this when you and your dog are walking as well.
Now often a person will telegraph their worry down the leash to their dog. So that might be a contributing factor and why he is aggressive sometimes and not others. In addition, it will usually get worse as instead of you just being a bit uncomfortable when a particular stranger passes, you might start worrying about how he will act. The dog will not be able to tell the difference between the worry and think it is the person and not that you are worried about how he will react. So it is important to not worry about how he will do and reward the desired behavior.
Once you have him pretty much trained and he listens consistently, I'd do a group class and then have him evaluated by a professional trainer (not an obedience trainer) if you want to train him as a protection dog. Most dogs will protect their owner if the situation ever really comes up, but a trained dog also comes with responsibility . A professional protection dog trainer will evaluate your dog and determine if his personality is stable enough to be trained for protection.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.