My theory. You failed to explain the situation in terms of what he cares about, and he didn't work out that connection for himself.
Remember. If you tell him that it will take you a lot of work, and won't ever work right, he will say, "Just another worker-bee moaning - when will they learn that they are being paid because it isn't supposed to be fun?" (But will be too polite to come out and say that to your face.) You are likely to get farther if you can honestly say, "This change will push out the deadline by at least 2 weeks, and the UI complexity will cause users to get frustrated at us." This he cares about. He may not believe your assessment, but he cares about the consequences described.
So add to your questions whether you made the problem description relevant to him, not you.