Kitty is scheduled to get "fixed," and you want to know exactly what happens when your cat is neutered. Don't worry; he should be just fine, and back to normal within a few days. That's a new and improved normal. Once neutered, male cats no longer have reasons to get into a lot of the trouble tom cats often find themselves in. He also won't develop testicular cancer.
Once neutered, your cat can no longer sire kittens. More important from a pet standpoint, he has no desire to engage in the activities necessary to father kittens. That includes roaming far and wide in search of available females, getting into fights with other tom cats and spraying urine to mark his territory. He's far more content to hang out at home and enjoy your company. If neutered before he hits sexual maturity -- prior to the age of 6 months -- he won't develop secondary sex characteristics. That includes a thickened face and additional muscles.
Before the Surgery
If you have your cat neutered by your veterinarian, she'll probably have you bring your cat in prior to the operation so she can conduct a physical examination and take a blood sample for testing. She'll have the blood tested for various condition that might contraindicate anesthesia or surgery. That includes kidney or liver disease, hypoglycemia or anemia. If you have your cat neutered at a low-cost spay/neuter clinic, the testing is usually done just prior to the surgery.
Your vet will advise you not to feed your cat the night prior to the surgery, but she might allow him access to water.
When Kitty comes in for his surgery, he'll receive medication to make him drowsy and relieve pain. He then undergoes general anesthesia. The area around his genitals is shaved and disinfected. The vet then makes an incision in the scrotum and removes Kitty's testicles. Most vets do not suture the incision closed, as this skin heals rapidly on its own. The entire process is quite swift. An experienced vet can neuter a cat in less than 5 minutes.
Your cat likely will come home the same day of his surgery. The first night, he'll be drowsy and disoriented from the anesthesia, but should be fine by morning. He might be sore for a day or two, but probably won't require pain medication. Your vet might send home a day's worth of pain medication, just in case. Some cats might mope for a couple of days, but many cats are pretty much back to normal quickly. If you notice any bleeding or swelling from the surgery, call your vet.