On a serious note, have you talked to any other owners who have their dogs neutered?
My eldest son has a cockerpoo and he had his bits off for medical reasons and it made him worse. I regularly walk him and have to be careful to where I take him. He barks at the slightest noise outside the house, he barks at strangers in the street and goes mental when he sees another dog in the street. He's a lot calmer off the lead but then I can keep him distracted with his ball but if a dog approaches he tends to snap at them. Its come to the point I will only walk him when it is quiet. Prior to the medical procedure, he was marvelous to walk.
It seems my son's dog is similar to what you are describing now and would discuss it further with your vet.