Yes-or-no question. He does everything in his power to not answer the question. She deserves an honest answer. Whether you don't want to date her
- because she's a minor and you're not,
- because you're hiding from her that you're actually ten years older,
- or because of reasons you can't tell her as telling her would mean the end of the experiment, dashing your career start,
you can hide all that and just say no. No, you don't want to date her, for reasons you would like to keep to yourself.
Not clearly saying no just keeps her hanging on to hope that there's a chance, when there really is not. And he knows there is no chance, because she'll ultimately lose her memories of him. For a supposed kind person, that's a really cruel way to treat someone you respect.