I own a fear reactive male. He is great with people, but not so good with other dogs. He is almost seven years old now, and I have spent the vast, vast majority of his life working with him on this issue.
He showed fearful behavior towards other dogs from the very beginning. It ramped up from there, mostly due to my poor handling.
I had him neutered at 14-15 months of age and it did not make his behavior any worse.