The first thing I would do is have him tested for a medical condition that might be leading to the sudden aggression. Some medical conditions can lead to sudden aggression such as thyroid issues. 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 this case, it is hard to determine which is the case as it might be a combination of. The fact that it is often directed toward smaller dogs tends to make me believe it is more of a dominance issue. I'd pay attention to the body language of the larger dogs that he challenges as if they are less dominant dogs, then I would be pretty confident, he is attempting to dominate them before they can dominate him.
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. This usually applies to dogs on leash but your dog also displays the behavior off leash. You might still be transmitting your worry with body language though. One way you can help alleviate your worry is to use a basket style muzzle on him during walks. They can still breath normally and even eat and drink with this style muzzle but they can not bite another dog. Most dogs quickly learn that they can not bite and stop the behavior.
You will need to be in control of your dog not only on a physical level but on a mental level as well, you must be the boss. 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. Right now it is directed at other dogs but he may get more dominant if you try and alter his behavior.
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. 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). This is standard advice I give all clients about furniture and dogs. It is relevant in any case of aggression and dominance aggression especially since the dog has to see you as the boss and listen to you.
You will need to start back up obedience training. 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. You can practice the commands he already knows or teach some new ones. The following site is helpful for this. 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.
Obedience training serves various purposes. It helps a dog learn what humans expect of them when they state a command which leads to self confidence and less fear. Each time a dog obeys a command, even if it is for a treat, it makes them a little more submissive to that human in the future which helps with dominance aggression. And since it is the leader or boss who is responsible for protecting the pack, if the dog is made submissive with training, you are responsible for protecting him, so that can reduce aggression due to fear and dominance. Of course, if he will stop and sit or lay down on command or come to you when he is charging or pulling toward another dog, it will help control him, so practice often and use a special treat like hot dog slices or liver slivers as treats. Keep them with you on walks so you can call him to you often and reward the behavior with those special treats. Don't switch them out with less messy treats as those are not as effective for training purposes.
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 trying to lunge at other dogs. You will need to practice this when you and your dog are walking as well. You will likely need several dogs and I'd focus on smaller ones first since those are the dogs he has the most issues with.
Another method is BAT. It is useful for this problem as well. You can read about this here:
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 additional 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.
You might switch to a front clip harness for him. It also moves a dog toward you if they pull but won't harm the dog's neck if they hit the end of the leash too hard. Instead of just the head, the whole body is moved around which helps. It is going to take a lot of work on your part to bring him around but combining these techniques and working with him daily should help you see an improvement pretty quickly though it may take months of working with him until he is ready for off leash, no muzzle walks.
Also try some agility training or tracking work with him. These dogs have lots of energy and while three walks may seem like a lot to us, a dog like this needs lots of mental stimulation. The obedience training will help, but some agility will help him burn off excess energy as well.
In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional in person behaviorist again. 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 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 .