Hello. This Is A Feline Question. There Is A Sweet

Hello. This is a feline question.
There is a sweet cat that lives in our yard and he now has a large cut in his throat. Yes, there is a scar and puss and the injury is be moving up both sides of his mouth. It's spreading.
Obviously, if we had enough money, we'd just take him to the vet, even though he isn't ours. However, we don't. We can't even to presently afford to take our 2 cats to the vet.
What antibiotics could I give this cat that might help? I have amoxicillin that I take when I see the dentist. Would that do any good for the cat? I have 500 mg pills. Please let me know if possible. Thank you. KeK