Thanks for the additonal information. It is helpful. The first thing that should be done is to rule out a medical cause for the sudden aggressive behavior though to me it sounds strictly behavioral. You can read about these medical cause here:
If there is no medical cause for the aggression, then it is strictly behavioral. Dogs are aggressive toward people for a variety of reasons. It might be that they are fearful of people and thus are aggressive before the person can be. In other cases, a dog is aggressive in order to dominate the people. Other causes could be that the dog feels they are the alpha member of the pack and as the alpha member they must protect the pack (you) from threats (people).
In addition, owners sometimes make the situation even worse by tensing up and worrying about what will happen. The dog senses the owner worry and feels that he is justified in his aggressive stance because you are obviously worried about the people. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the people.
For a dog like this, total control is necessary. This means not only physical control but on a mental level, you must be the alpha. To accomplish this, you will need to have the dog wear a basket muzzle anytime he is not in your own house or yard. This will not only prevent bites but also allow you to feel more at ease when walking him or having him loose. If he is on a lead there is no reason why you should allow him close enough to jump or nip another person. That is part of the reason dogs are leashed so you have control over them. So until this is resolved, he should not be allowed close to people when walking.
You will need to have him obedience trained. If you can, I would do group classes (with the muzzle) and let the trainer know of the problem your dog has.. Before you can get into classes, I am including a link to a great site that teaches you how to train your 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.
This obedience work establishes you as the alpha member of the pack and as alpha it is your job to protect the dog and not the dogs job to protect you. Now the dog will still protect you if something should happen because you will be protecting yourself and as part of the pack, he will follow your lead and protect as well. But if you are calm around someone, he will see that you are ok and there is no need to help. This is why being the alpha member works well. Obedience training also helps a dog gain self confidence and what to expect from people so it can reduce fear aggression as well.
You will also want to keep a leash on him at all times initially to grab if he should disobey. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you.
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 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 person 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". Since he tends to try and jump first, at the first sign of jumping, a correction should be given. It shouldn't take your dog long to realize you will not tolerate the aggression or jumping and that if he ignores the person, he gets treats. Once this happens you can repeat the training moving the person closer until he is no longer trying to jump up or nip at people. You will need to practice this when you and your dog are walking as well.
Bat training may also be something you want to investigate and try.
.Part of obedience training is also the fact that your dog should be paying attention to you and not other people. They should also obey you immediately, so if he went to jump on someone and you said "down", he should immediately lay down or if you say sit, he should obey immediately thus also stopping the behavior. Constant reinforcement of the commands through daily practice is important to achieve this type of obedience. For your dog it is important that you achieve this level.
In addition, if the situation is not improving with the obedience training and other training techniques you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
Unfortunately, at this point, I would seriously consider having a behaviorist anyway available to evaluate the dog and determine the aggression level. In the meantime, institute the techniques I have recommended. It is very important to get this situation under control. In many areas, a dog that has bitten a person more than once is automatically euthanized.
I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .