This is Dr. Christian. Is there any bleeding or discharge? Any redness around the wound or swelling? Is he an outdoor cat? Can you tell how deep the wound is?
There are a couple of possibilities based on what you are describing.
Warm water is all that is necessary to keep this clean. You can apply neosporin to the area but that will not help any infection deep under the skin. I would recommend taking him in to your vet in the morning to be seen. He'll probably need oral antibiotics in order to get this to heal, especially at his age. If you have more questions please let me know.
You are welcome. Good luck.