I agree with Eddy, if it’s gotten worse with time instead of better, you do need him to be seen. It could be ringworm (a fungal infection, so nothing to do with worms). You can of course rinse it with saline (normal saline eyewash for humans) meanwhile and gently pat it dry with something clean and soft. You can’t use cortisone on a bunny exept in extreme cases and as ordered by a vet, so he’d need something else for the itching. (Rabbits are extremely sensitive to corticosteroids).
If it’s an infection it could spread.
Here’s a warning we always issue when the subject of possible external parasites comes up: Do NOT treat a bunny with any compound containing fipronil. It’s deadly toxic to rabbits. Fipronil is sold under a wide variety of names (Frontline is probably the best known). This doesn’t really sound like parasites though since it’s localized to one spot on one ear. Could you get us a picture?