I was hoping to get some information back. In most cases dogs who are aggressive toward other dogs can be taught to behave around other dogs. They may never like other dogs but if you can stop them from lunging, growling or even attacking other dogs then it is a plus.
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. Dogs are aggressive toward other dogs 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. 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).
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. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog.
For a dog like this, total control is necessary. This means not only physical control but on a mental level, you must be the boss. 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 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. You didn't mention this so hopefully that isn't a problem yet.
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. Attach a leash and use it to remove him from the furniture. Give a correction in the form of a quick tug and firm "NO" when he attempts to get on and a treat when he starts not trying to get on the furniture. Thus you are providing negative reinforcement for the getting on the furniture and positive reinforcement for the desired behavior (not attempting to get on the furniture).
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. It might take you a few months of basic training before he is ready for group class. . Before you can get into classes, 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.
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 with a dog 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 their dog 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 other dog 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 other dog closer until he is no longer reacting to the other dogs. You will need to practice this when you and your dog are walking as well.
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.
I hope this information is helpful to you. 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 . If you do find this helpful, please take this opportunity to rate my answer so I am compensated for my time.