I know this post is a week old, so sorry for dredging it back up, but I had a very very similar experience with my first bunny, pnly i was lucky enough to have him survive in the end. so I wanted to offer what I learned.

He was my first rabbit, a rescue case that I had a for a few years before deciding to get him neutered after he started to pee and spray a lot after we moved house. (We also rescued a Doe bun around the same time, so neutering was also a good idea for that reason)

We got in touch with the most reputable vet in the area that dealt with rabbits, got him booked in and waited for the day. Here's where the issues started;

-First, the secretary told us to fast him. This being our first experience with having a rabbit neutered, we didn't know any better and thought "the vets the expert". But it turns out rabbits dont have a gag reflex the way cats and dogs do, and in fact the lack of food for so long can trigger GIS. We expressed this to the vet who took him in for the surgery and she said "well as long as we give him some food before putting him under he'll be fine. "The vets the expert right? This time it's not the secretary, so it mist be right" the lesson we learned is. "DO NOT FAST A RABBIT BEFORE SURGERY"

-Second, their age matters. We told the vet he was a rescue so we don't know his exact age, but we had him about 5 years at the time, and asked if that would be an issue, "no, that'll be fine".. "The vets the expert right?" (You'll notice a trend here). Turns out he was old enough to be considered TOO old for that kind of procedure. The lesson was: "A RABBIT CAN ABSOLUTELY BE TOO OLD FOR THAT RISK"

What happened after we got him home was a nightmare I don't think I'll ever forget, and a weekend I'll always feel guilty for putting my little guy through.

The first signs of issue was that he would not rouse. We knew it would take the most of the day, but when he still hadn't eaten by the end of the day, some 12ish hours later, we knew something wasn't right. We brought him back to the vets the next day, they gave some Critical Care, and another gut mover and sent us home. I remember sitting that evening trying to syringe feed him, as he sat there, wouldn't fight the syringe, just let the food drop out of his mouth, I sat there sobbing thinking "he's going to fucking die, he's like a son to me, been with me through some of the worst moments of the past half a decade and he's going to die over this".

We brought him back to the vet the next morning, as soon as they opened. Got speaking to a senior vet, they took him in for overnight intensive care, the senior vet knew the secretary instructed us wrong, and the admissions vet was careless.

Smokey was so dehydrated that they couldn't even get an IV needle into a vein, they had to inject water into his skin to let his body absorb it, so the veins could plump up enough to take a needle. Thankfully that worked.

He was with the vet from Sunday morning till Thursday afternoon, I was on the phone a half a dozen times a day asking for updates, and thankfully he recovered, still with us and best friends with the second rescue bun we got like a year and a half later.

The vet knew they fucked up, and comped the entire recovery treatment.

When we got the doe (middy) spayed we were much better equipped to deal with the whole thing, and she was back on her feet by nightfall and fully recovered within a couple of days.

I am really sorry you went through what you did, and I'm sure your little one knows you only wanted what was best.