He showed it especially around other dogs but seems worse now he’s been neutered. Some dogs he ignores but others it’s like he’s manic on a leash. The moment he sees a dog and they’re fixed in his eye line he becomes tense and his whole body tenses up.
Unfortunately, the neutering has probably made the behaviour worse as it gets rid of the testosterone in their systems. It's not recommended to neuter male dogs that show any kind of reactivity or nervous behaviour, but unfortunately many vets won't question it and will just go ahead if it's what the owner wants.
You can work on it, though - he's certainly not a lost cause! Find his tolerance level for other dogs (ie. the distance he can be without reacting) and practising getting him to sit and focus on you with a treat. Eventually you'll be able to move closer and closer, and the treat can be phased out as he'll learn to ignore the dog to get treats.
But this can take months and months of hard work and it's very easy for setbacks to occur. I'm speaking as someone who probably shouldn't have had their dog neutered due to reactivity issues, but I'm there now so it's all about working with what I've got. He's MUCH better than he was but it's still a work in progress.
His reaction isn't aggression as such, more fear and curiosity I suppose. Once he's met the dog he's absolutely fine and he'll happily play. He's also absolutely fine off the lead - leads can increase reactivity as it stops them behaving naturally.
Be careful when getting a trainer - anyone can call themselves a trainer or behaviourist - make sure they're accredited and only use positive training methods.