Welcome!
I'm concerned that this was deliberate! did it look like a clean cut? If not, are there racoons that could have fought with him? (is he not nuetered so likely to fight?) If he's nuetered, and it looks like a wild animal did it (well nuetered or not i suppose, you might want to watch out for signs of rabies but don't get over alarmed about it either. It's way less prevelent than the health dept would lead you to believe. Just be on awares, that's all. If it looks like a human did it, do you have any idea who?
Any way a shelter would take him? (you'll have to wait till he heals now or he'll be quarenteened). Well, the 1st and best thing is to get an antibiotic, maybe your vet doesn't even need to see him. If it's too costly, ask your vet for the number to a rescue group that can get you a vet visit and meds at a discount rate. They are out there. Keep us posted.