Do you know what an Elizabethan collar is?
Better known as a buster collar in the pet world, you mostly see them worn by dogs but I had a tiny one for the first buck I got neutered, he wouldn't let the area be.
Some boys are good and don't bother the area. Most vets only issue cats with collars automatically so you can ask your vet for one if you think Spot will be be naughty. Apart from that, just watch him carefully for the first 3 days or so and if he lets you, pick him up and and have a wee look to see how the wound is healing. I don't know about other vets but my mine always includes a free check up ten days after the procedure just to make sure everthing is healing properly.
Oh and something else. If Spot sprays and or humps, this behaviour will change after the castration but not right away. The hormones which influence this behaviour can take 6 months in some cases to die down. But the randy behaviour does stop so don't worry about that too much.