I had a cat with an sore that didn't heal for a year or more, we tried antibotics which you can get at a pet supply store wtihout an rx. we did end up haveing to amputate the tail, I am sorry for that, but its better than putting the cat down.
Let me add that it is possible he doesn't need surgery. It is possible to pack/ cover a wound, give antibiotics, and allow it to heal as best it can. The result is never pretty, but survivability is high.
Is there any infection? I wouldn't be worried about rabies. I'm wondering if it's an abcess that drained. An especially large one could leave this situation as described (a wound of that side would likely be infected).
The fact that he came home at all is possibly a good sign. If there's no infection, you can try cage resting him and feeding him with one of the milk supplements for sick cats (not the treat milk)
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If your cat is incapable of eating/drinking, or it infects and you can't afford a vet, then I'd consider having him put down/ killed.