Cats' Health Issues

Welcome to the board. Unfortunately with a wound that size, it needs a drain and sutures. Cats skin heals very quickly and if that closes up and there is still a pocket underneath, an abscess will form which will require surgery anyway. They can also get very sick if the abscess forms (fever, not eating, etc.) I highly recommend you call your vet and tell them your situation, many times if you are up front with them, they will keep the cost down and try to work with you on the payments... at the very least, your kitty needs antibiotics to help with the healing process and possibly a drain (without seeing it I can't tell you if he would need that too...)

Good luck!

I know vet bills can be expensive, but you might consider asking a vet to work with you.  If you have a regular vet they will most of the times.

He most likely is continuously licking it so it won't heal.  You could get him one of those head collars to keep him from licking it, but cats hate them.  You could also maybe wrap a piece of gauze around his middle, but he'll probably hate it and try to get it off.  Also try neosporin as that is a safe cream to use on cats.  If the cut is deep though, it might require stitches.  If you don't get him to a vet, it could still become reinfected.

Cats have a tendency to fight outside.  I would recommend keeping your cat inside to avoid any further issues, not to mention the higher risks of getting hit by a car, taken in by animal control, attacked by a dog, etc.

I understand it is extremely difficult to keep the cats inside when they are indoor/outdoor cats.  I've taken in some strays throughout the years that I wanted to make strictly indoor cats, but they would have no part of it.  What are your living circumstances like?  Is it possible to just let him out on a deck, or a backyard, that is fenced in?  There are also some products available that will help to keep your cat nearby:

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=16126

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=3100

Though I know these are expensive, you might be able to find similar products for cheaper somewhere else.  Our deck is about twenty feet off of the ground and never had steps the backyard and our cats used to just love sitting out there, even though they couldn't get anywhere.  Maybe even screening in a little portion of an outside space might help, and you could also use a dog crate to let him go outside without running off.