My dad keeps doing this shit with his diet sodas. How do I get him to stop?

Obviously not my photo, I don't upload photos of myself or my environment. He will not stop pouring sugar into his diet sodas. I've tried reasoning with him, but he gets pouty and insists that the diet ones (even Coke zero) taste worse than the ones with sugar. He's got a family history of diabetes. Do they really taste that different? Is there a way we can get the sugar out of a regular coke can or bottle? I'm pretty sure he wouldn't notice, I think he's just placeboing himself into thinking diet Coke tastes worse.

Obviously not my photo, I don't upload photos of myself or my environment. He will not stop pouring sugar into his diet sodas. I've tried reasoning with him, but he gets pouty and insists that the diet ones (even Coke zero) taste worse than the ones with sugar. He's got a family history of diabetes. Do they really taste that different? Is there a way we can get the sugar out of a regular coke can or bottle? I'm pretty sure he wouldn't notice, I think he's just placeboing himself into thinking diet Coke tastes worse.