I had my two previous piggies neutered.
They were both fine after surgery but I was advised to place them on towels or vetbed instead of hay. Their wounds could get spiked with hay and infect the area so just limit their hay to their racks. Towels need changing at least twice a day as you don't want them sitting on wet towels, again for infection. Vetbed would last longer though. Keep them nice and warm. Towels etc need to used for about 2 weeks.
My boys' appetites were unaffected by the surgery, still greedy as anything. Stitches normally come out about a week later. They don't like that bit! THey squirm about and squeak.
Now, the most important thing is check, check, check the site every day. Even after the stitches are removed, keep a close eye on their testicle area, in fact make it part of your daily checks. One of my pigs,. Gordon, was fine then one day there was a huge golf ball swelling. This was an abscess. It is quite common for an abscess to develop and they are nasty. Poor Gordon did not make it, but that was mainly due to the bad advice from the vet with wrong antibiotics etc.
So my advice is, keep looking for any swellings down there. Funily enough, they still retain the appearance of testcicles even though they have been neutered, I was surprised at that. But this is the main thing to look out for.