Thanks for the additional information. It is helpful.
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).
Before your older dog died, the job of protector or top dog was likely the older dog's. Now this dog may feel they have to be the one to ensure the pack is protected. the pack would included you.
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. There are even articles about leash aggression. Often just letting a dog run loose stops the behavior. So try to be as calm as possible when walking him on the leash. Don't tense up or pull the leash tighter.
Step up obedience training. 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) as well. It is outlined below.
When training, be sure and use a tasty treat like hot dog (vienna sausage) or liver slivers. These are high value treats. 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. I'm glad you are not allowing him up on the furniture.
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.
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.
The bat method may work as well. You can read about that below.
In addition, if the situation is not improving using the techniques I describe, 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 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 .