You can also get OTC ear mite medicine. It's not expensive, but it's a bit messy to work with.
It could also be the infection is back. Try the medicine...but if it doesn't work, I'm afraid you'll need antibiotics. Sometimes secondary infection pops up in the ears or vaginal area of a cat as a side infection due to a primary other infection source.
Your cat may have ear mites. My cat did the same thing by shaking his head and even scratching at and in his ear and I took him to the vet and he had ear mites and they gave me some medicine for that. If it's not ear mites, then the crusty stuff is probably annoying him and that's why he's scratching at it, to try and stop the irritation that's causing the itching. Like you said, his ears are very tender so it makes sense. Maybe you could try using a dry paper towl and gently clean his ears every now and then, that may help.