My neutered male cat has a swollen, red, crusty nipple. What's wrong?

It would be hard to determine what the growth is without having it examined by a vet. Infections such as abscesses often are hot to the touch, painful, swell rapidly and may ooze debris from them if they pop open. Tumors and other growths may have different rates of growth and may or may not be bothersome to your cat. It is best to bring him into your vet to have the area looked at, and it may be drained or aspirated to take a sample for sending off to the lab. If the area is an infection, it can be drained and antibiotics given to prevent its return. Until you can get to the vet it is best to not allow your cat to lick or chew at the area with the use of a T-shirt or Elizabethan (cone) collar so that bacteria or a secondary infection are not introduced.