Are you talking about an "inner ear infection" or just an infection in his ear? Is it possible that he scratched his ear and it is a bad infection that is not actually behind the eardrum? In humans, ear infections are usually caused by "junk" backing up especially in small children when you have a cold or other sinus infection.
Maybe he scratched the inside part of his ear with his foot and it had some germs or something on it. The culture will tell you what it is. And whether the antibiotics are the right ones.
