The first thing that should be done is to rule out a medical cause for the sudden aggression. You can read about these here:
If there is no medical cause for the aggression, then it is strictly behavioral. In that case, there would need to be a lot of changes made in order to bring this dog under control. Dogs are aggressive toward other dogs and people for a variety of reasons. It might be that they are fearful of other dogs and thus are aggressive before the other dog can be. In other cases, a dog is aggressive in order to dominate the other dogs and be the alpha member of the pack. The same thing applies to 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 (other dogs and 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 dog or people. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog or the people. This reinforces the behavior they are showing. Anytime they exhibit this behavior or even fixate on another dog or person, a correction is in order. A correction is a short tug to break their concentration and a firm, low toned "NO".
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 may want 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. If he is not neutered, have that done. Many male dogs do challenge their owners at around 18 months. The challenge may be a blatant disobeying of a command or even as severe as a growl. If you allow the dog to disobey you or growl, this teaches the dog that he is more dominant and he starts to get the idea that he is the boss and thus he is in charge of the humans and as alpha it is his job to protect you and the family as well.
Many dominant dogs are described as well behaved until you try to get them to do something they do not want to do, and then they reprimand you either with a growl or bite if you don't heed the growl. Things like taking away something they want, making them move when they don't want to, waking them up, etc can cause them to reprimand (bite) you.
Dogs that are allowed on furniture (even if put on the furniture) tend to feel that since they are elevated to your level or higher if on your lap, they mentally feel elevated as well in the pack order and thus are the boss. Keeping them on the floor can help lower them mentally back to a submissive position in the pack. So the first thing is to not allow him higher that the humans or even on the same level. In addition, humans shouldn't be on the floor with him either. A small short stool is enough to keep them higher than the dog when petting the dog.
You will need to have him obedience trained. If you can, I would do group classes (with the muzzle if necessary) and let the trainer know of the problem your dog has. I know you are working with a private trainer, but the dog is displaying the behavior around strangers and other dogs. In private sessions, the dog won't be exposed to those triggers. However, the obedience training from private sessions will at least help establish you as the alpha. I do suggest all family members be active in this training. It might take you a few months of basic training before he is ready for group class. . Before you can get into classes, I am including links to a couple of other sites that teach some good methods of training. Be sure and read both.
The following site is helpful. Be sure and click on the link to the left on obedience. 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.
You will also want to keep a leash on him at all times initially to grab if he should disobey. I think you will be pleasantly surprised at how well your dog does with training. 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.
In addition, if the situation is not improving using the techniques on the previous website, 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.
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