Does castration curb aggression

Possibly not, because aggression is almost always rooted in anxiety and by castration him you would take away his ”brave” testosterone.

When he feels anxious about other dogs he has to put on a big display of 'I'm big and scary, don't mess with me'. And it generally works for dogs, the other dog usually does retreat so it becomes a reinforced behaviour.

He will have an invisible radius of space around him where he feels secure. It's called flight distance, anything within that space triggers his fight or flight stress response, which you may have heard of. Find out what that is and keep him far enough away from other dogs that he is relaxed. Reward him generously for being calm, you are trying to develop a Positive Conditioned Emotional Response. Google +CER for more information, and this link explains it more.

Care for Reactive Dogs

Gradually, over weeks and months, not days, work on reducing the distance. This may mean you have to be selective where you walk - choose places with good visibility so you can give other dogs a wide berth, or where you can turn and walk away easily. But - be aware that if your dog has had a stressful episode the stress hormone can stay in the body for some time (days) so a distance he was comfortable with the day before might be too close that day. So the safe distance can change, watch his body language.

Trainers describe behaviour like this with reference to the three Ds. Distance, as above but also be aware of Duration (your dog might be tolerant for 10 seconds, but not 15) and Distraction - how distracting the stimulus is; a calm dog might not trigger any reaction at a given distance but a bouncy one might.

Alongside that you could train a 'watch me'. As your dog looks at you, mark and reward the behaviour. Ask for longer periods of watching. Then if a dog approaches, after you have worked on the distance issue, you can get your dog to focus on you and not the other dog. BUT - some dogs find this scary as they cannot see the thing they are anxious about so you need to judge your dog.

If you are still considering castration, there is a temporary implant called Suprelorin, which would show you whether there would be any change in him, but wears off so you aren't stuck with a permanent change if you don't like the result.

 

Umm....maybe not in a way you are thinking but castration can have a link to a changing behaviour. It is possible that it ain't so much that other dogs give off testosterone but the changed hormonal balance or lack of them can affect dog's own behaviour towards others. Lack of confidence (fear) leads towards aggression...
There is scientific data out there to support the idea of hormonal issues causing lack of 'good' behaviour with dogs, some vets don't agree with them or choose to ignore them altogether and then there is us 'mortals' who have to make all the decisions for the 'best of our pets'
But what ever has caused the change, you will still have to train him to cope with his reasons.
Link to articles about different types of aggressions ...Aggression
..and about neutering...Neutering Dogs. In Depth

 

You are aware that growling is NOT aggression. It is a warning to other dogs/humans that the dog is uncomfortable & is warning other to keep away. Barking isn't really truly aggression either. I had GSDs for many years & they are a very vocal breed that enjoys a good bark & often greet knowm dogs with almost frenzied barking.

Castration does not "cure" aggression, the only thing you can guarantee castration does,is eventually render the castrated dog infertile.

Has your dog had any encounters with other dogs, even if there has no contract between the two dogs. I have a BC who ws the victim of two incidents involving a black adult Irish wolfhound. Since the incident over 12 1/2 years ago, he hates giant black dogs & it triggers apparent "aggressive" behaviour, it isn't aggression of course, it is a pure defensive behaviour.

My two younger dogs( mongrels and castrated)sound as if they are killing each other when they play they bark & growl but are never aggressively & they have never hurt each other