Dr. F explained that he went in, and cut out infected/dead tissue, all the way back to what appeared to be the root of infection. He said it started at the point where sperm would enter the teste. Upon further research, I think he was talking about the ductus deferens...? Said it was more likely infection that had stemmed from his neuter in March, but we couldn't be certain.
But the good news is he thinks he got it all, and there was enough healthy tissue that he was able to close him up without trouble. I was concerned since the infection was so close to the rectum, and when Dr. P initially was talking about a fistula there was some worry that he would start passing poops out the hole beside his rectum. But he's been having healthy, normal poops since coming home, so yay!
This morning he was very impatient to be let out. I let him out on the bed to give him his metacam and bactrim, and snap a picture. As soon as I snapped the picture, he went bounding away and scrambled down to the floor, and then proceeded to run around like a crazy rat. This is his normal a.m. routine, so I guess he's not feeling too much pain or discomfort.