[Curls]

If I'm reading this correctly, you are really dealing with three issues. One is that your dog needs more exercise and stimulation than you can provide by taking him on a regular leashed walk. The second is that your dog is fearful of other dogs. The third is that he handles his fear by becoming aggressive.
First, dealing with his exercise and stimulation needs. Have you explored any kind of dog sports such as carting or skijoring to give him a cardio workout? I also find that trick training helps a lot with keeping my dogs sane. Practicing sitting, crouching, bowing, walking backwards, etc. exercises both their mind and their body. Think of it like yoga for dogs.
Second, regarding the fearfulness, do not let him interact with other dogs. Keep him out of environments where there are loose dogs. Get his attention and lead him away before the other dog comes close. Also, make the sight of another dog a cause for celebration. Keep treats in your pocket: lots of treats. (I've also heard a suggestion to use something like liver or cheese paste in a squeeze bottle, but I've never tried it myself.) Start shoving treats into your dog's mouth as soon as you see the other dog, before he starts reacting. I don't mean one or two treats. I mean handfuls. Keep stuffing treats into your dog as long as the other dog is within view. If your dog is too upset to eat the treats it means you are too close. Move away from the other dog and keep feeding treats. This treat tactic won't produce immediate results; it will probably take a month or two of distant dog sightings before his attitude starts to soften a bit. Eventually, though, he should start turning to you when he sees another dog. Both the reward (the treats) and the task (staying by your side within range of the hand that is dispensing treats) give him something to think about other than the strange dog.
Third, since he starts fights, take steps to ensure he doesn't harm another dog. Again, keep him out of environments where he can come close to another dog. You may also want to consider a basket muzzle to prevent him from biting another dog. Aside from the safety factor, the muzzle will make your dog look more scary. The visual effect might make other dog owners a little more careful about letting their dog run up to yours.

 

[Curls]

I will try more intenselly giving him treaths when we spot another dog. That is usually not a problem though passing other dogs on the street. He does get a bit interested in greeting the other dog but it isnt that bad. The stress seems to start when he is actually greeting the other dog.

Would it be a good training if I managed to get another male dog to be close (some meters) and simply practise calmness with my dog? Is that a good way to make him feel calm around other dogs?

Regarding the last point: he has never harmed any other dog before. If I am there I manage to break up the fight within seconds fortunately. Wont work from a distance though..

I think some of these questions are best handled by a consultation with a certified animal behaviorist. I'm not one, nor have I seen your dog in person. I'm just a person on the internet who has owned some reactive dogs. I'm offering advice based on what helped my dogs. Some additional thoughts:

1) You don't want to put your dog in a situation where he is practicing the behavior you don't want. He's probably giving off lots of unhappy signals before a fight ever starts: stiff body posture, licking his lips, etc. We humans are quite bad at reading subtle dog signals. A danger of deliberately exposing your dog to something that bothers him is that he will rehearse the emotions without you realizing it.

2) Sometimes deliberate exposure to a frightening thing makes the fear worse. It also destroys trust. Imagine if your spouse tried to help you get over your fear of spiders by borrowing a friend's pet tarantula. Every day your spouse would bring the tarantula into the kitchen while you were eating breakfast: "See, she's a very sweet spider. So cute, so fuzzy, such lustrous black eyes..." Most likely you would not develop any love for the tarantula. You would start dreading breakfast. You would be torn between wanting to squash the spider and knowing it's wrong to squash somebody else's pet spider. After enough ruined breakfasts you might threaten a divorce if that spider didn't go home to her proper owner immediately. You might even learn to hate coffee, because every time you smell coffee you think of the big scary spider in your kitchen.

3) Consider the impact of a dog fight on the other dog. You know your dog gets upset. What about the other dog? Will the other dog leave the fight a little less happy, a little less friendly, a little less trusting of other dogs? Is it fair to other dog owners to risk an encounter that might permanently worsen the way their dog interacts with other dogs?

 

[Curls]

Ideally you want to react before your dog reacts. The treats are simultaneously a distraction, a calming signal, and a reward for staying calm. Watch for other dogs, get his attention (moving him further away from the other dog and blocking his view with your body if needed) and start shoveling treats into his mouth before he reacts.

Theoretically you should not give treats when the dog is behaving badly. However, there is also a concept of "reward a good attempt." So, when I move my dog 10 meters further away from something and he changes from barking to whining, I will give him treats. 1) Whining is an improvement over barking. It is a good attempt, and I want to reward that. 2) Being willing to eat the treats tells me that he is calming down; we are at a good distance. If he won't eat the treat I know he is still too upset to think or to learn anything, and we need to move further away.