Thank you for being responsible and having your cat neutered
He will be fine, when I took Sonic in he was there for around 7 hours (during which time he went through a pre-op examination and heart-check, anaesthesia, the surgery itself, and observation and care while he came out of the anaesthetic), and when I brought him home he hopped out of his carrier and ran around as if nothing had happened.
He looked a bit sore around his scrotum, but there were no stitches - just tiny incisions sealed with glue, he didn't need to wear a cone, and he didn't need a followup visit to the vet. Kittens heal quickly, and after 2 days you couldn't even see he had anything done down there, it no longer even looked sore.
Even when my other cat Radar was done he bounced back fine, and he had to have his umbilical hernia repaired at the same time (which I have been led to believe is worse surgery than a spay), he had 5 stitches in a 2 inch incision down his abdomen which looks huge on a tiny cat and he didn't need a cone or special treatment either, he was a bit woozy the first night we brought him home because he had to have stronger sedation but the next day he was back to normal.
I'd ask your vet about the overnight stay, mine doesn't do that - it's drop them off in the morning and collect late afternoon, but every veterinary practice has their own way of doing things and it may just be that they like to keep them under observation until they are active again following surgery, but whatever the reason your vet won't mind you asking.
They should have advised you to remove all food and not feed him the night before, this is because the anaesthetic can make them vomit and if that happens while they are on their back under sedation they can choke on it, so make sure you strictly follow the vets instructions about removing food before he goes in for surgery.