I have some positive experiences - maybe that will calm your nerves just a little bit. We got both our boys neutered last Wednesday so I know exactly how you feel! I was terribly nervous too.
Both our operations went with 0 complications and both our boys are absolutely fine. They were eating pretty much as soon as they got back home, they didn’t have any mushy poos, and they barely lost any weight (both about 20g). I offered them some recovery food the first evening after surgery to top them up. They were both a little groggy and sleepy so didn’t eat as much as they usually would and I just wanted to make sure they didn’t lose any weight. But since then they have been tucking in as always, they‘re happily popcorning around, flirting with the ladies through the bars and giving the best cuddles
The only thing I did was clean their litter trays and put fresh bedding in to minimise the infection risk and I’m weighing them daily to make sure I don’t miss any sudden drops in weight.
I know it’s easier said than done but I’d try to not worry too much, these little men are a lot tougher than we think. We‘ll definitely keep all our fingers and paws crossed for your little boy tomorrow and we hope his little operation goes smoothly and he recovers quickly ❤