I can tell you I have never ever seen or heard anyone say their dog has BECOME aggressive after neutering. I have read it, but I personally don't believe it. I will say, from personal first hand experience with one of ours we have now, Shadow. We had planned on leaving him intact, but the older he got the more aggressive he became. He was randomly jumping on and fighting all of our other dogs, even females. We had been bitten more than I can even remember separating them. This was happening on average three times a day, then some aggression was starting to be shown towards us at random times. By now he was three years old, and we decided to give him one chance by neutering him to see if it help tone him down. If not we were going to have him euthanized. It took a month to see a change, but ever since he has gotten better and better. It's been almost a year, and he is a different dog. Still has some "growly" moments, but no more unpredictable attacks. Literally, it was a life saver.
If of as many dogs went the other way and became as aggressive after neutering as has been mentioned, there would be a lot of mean dogs out there! Neutering is done every day and this just isn't a common thing.
Now, are there health benefits to leaving them if possible? Absolutely! The benefits outweigh the negatives by far. You can google all the benefits to leaving them, so if you are able, and your dog is manageable I would leave him.