Neutering and dog aggression

I have a 4 1/2 year old pitt/ pointer mix with dog aggression. He is friendly towards humans, other types of animals, and once introduced, all female dogs and some neutered males. His aggression comes out when he sees a strange dog anywhere, no matter how far away, he becomes very focused and it is very hard to get his attention and make him listen to commands. He used to be very uncontrollable and flip out and lunge bark snap and growl (I used to use a pinch collar and scold him when he growled). Now he will sit or lay down, but he will shake violently and growl quietly (I have switched to a headcollar [Halti] and praise when he doesn't bark) unless the dog gets too close then he will attempt to lunge and bark. I have made a lot of progress and am proud of him, but he is an emotional support dog and I feel this is undesirable and gives this type of service animal a bad name. I have not wanted to get him fixed until he was completely trained, feeling that it would be a cop-out. But recently I've been thinking of getting him fixed for cheaper licensing fees and health reasons (also a law was recently passed in my county that requires dogs over 6 months to be fixed). But I've heard that fixing a dog that old can make them more aggressive instead of less, and after all the work I've put in him that worries me, being set back two years on his training is not appealing. Also I don't think all of his aggression is from hormones. When he was 6 months old he was attacked by a pittbull that tore his front leg open, and over the years multiple dogs have ran up to him an attacked (I live in a poor town and there are a lot of strays and dogs who jump over their fences and jump back). He flips out the most on pittbulls and german shepherds, which are the two breeds that have attacked him the most in his life. Basically I am wondering what are the chances he will become more aggressive instead of less?

[Aleda M Cheng, D.V.M., C.V.A]

It sounds as if you have been doing a wonderful job training your dog despite powerful odds.  It is a difficult feat especially with your dog's unfortunate history of having been attacked by other dogs.  Dog's do have long memories.  However, I think neutering will help.  Statistically, it has been shown that male dogs are 60% or so less aggressive towards other dogs once they are neutered.  Additionally, potential attackers may avoid your dog a bit more once he is neutered, since they will no longer feel that he is competition for mating.

Testosterone will take a month or so after neutering before it goes down low enough to change any behaviours.

There is no way to know positively if neutering will be effective, but it is worth a try.