You did say he was bleeding from the mouth...that means he is likely to get a nasty infection in his HEAD, near his BRAIN, & affecting the only mouth he has to EAT with.
Ignoring it doesn't sound like a good plan...just saying. Cats carry far more nasty germs than most animals, look it up.
Antibiotics are MOST effective when started EARLY, before the infection takes up residence. Don't say no one told you.