Thanks for the additional information. It is helpful.
There are medical reasons for aggression and though I don't think that is the cause, I'll give you sites that discuss these causes. http://www.petplace.com/dogs/medical-causes-of-aggression-in-dogs/page1.aspx
Dogs are aggressive toward other dogs for several reasons. They might be fearful of other dogs and thus are aggressive before the other dog can be. In other cases, a dog is Another reason might 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). Jealousy usually falls under the alpha umbrella because he doesn't want other dogs near his person.
Owners often 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 that worries you.
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 haven't' mentioned anything like this but you might keep an eye out for that type of 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 I would stop allowing him on the furniture. You can 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. Make it a special tasty treat like a hot dog slice. 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 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, I am including a link that is great at teaching you how to teach 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.
Obedience training teaches a dog 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 that he is reactive with 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.
If he isn't lunging at them yet, have him sit a little bit away form the other dogs so they are not interacting. Or you can let them touch noses and then move him away and have him sit and reward him for the interaction with no reaction. Keep him away form the other dog after the intitial contact. That will help him see that you can protect him by keeping the other dogs away from him. Dominant dogs do not like other dogs smelling them or putting their heads over their neck.
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.