Neutering is less difficult/invasive surgery than spaying. I'm surprised the vet won't release a male the same day; most of ours here in the Hudson Valley do. Females stay overnight as a rule.

Some vets spay/neuter when a cat is at least 2 lb, approximately 8-10 weeks. This is not done in this area, but I think some of the shelters in New York City do it.

Good luck with your little guy, and thanks for doing your part to protect the cat population.