I'm sorry to hear that your oldest took a turn after neutering. Although that can happen, most cats keep the disposition they were born with unless something in their physical environment causes them to change, i.e. hearing thunder and becoming afraid of the loudness and vibration, being attacked by another cat or animal, mistreatment by humans, etc.
Have you asked your Veterinarian about this? A professional could check your cat to make sure nothing is physically wrong with him to cause him to act this way. Also, was he treated roughly before or after surgery, frightened by another animal at the Vet's office, did he have enough anesthesia and pain medicine so that he didn't suffer before during or after the surgery, etc.?
Also, how long after his surgery did you notice this change? Sometimes an animal needs to be given time to recover from such a big event like this but if he continues to act this way after a week or two I would consult your Veterinarian about it.
In the meantime, best of luck to you and your cats!