to look like this ?
Agreed all around.Slip ups do happen. He looks a little full figured. That said, I also have one that seemed to have a little leftovers. Is he trying to mate or just grouchy? I assume you have his paperwork on the neuter. If a vet says he was not fixed I would get that documented and get a return on your cash! A lot of TNR places have volunteers working. It could have been an honest skipping of one cat on their extensive list for that day. They often do a great many in a day. I certainly would check to prevent unwanted pregnancies.
He was about a year old when I took him in. I can't say that I noticed at the time if he looked done.Some males don't look like they're neutered, especially if they were past puberty when it was done. Did he ever look like surgery was done? Did he have visible stitches/glue/incisions?