Try walking away from him, at least to Gile's Farm and then run back and wait for him to completely talk about his little association of cheese barrers, once he finishes then hold (A). He is probably stuck in a conversation that never finished, when you try to do things infront of a Demon Door while it's talking it usually doesn't pick up on what you're doing, so like I said, just travel a ways away and returned then wait.

"Wisdom is knowing what to do next; Skill is knowing how to do it, and Virtue is doing it."
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