Why doesn't she just... allow him to try to dispel the spell then? If she really isn't under a spell, it could be solved like that. On the other hand, this is a rom-com. Why would it be that easy?
I think part of him is worried that if he dispelled the charm spell on her, she'll go on a rampage and destroy everything in sight, just as what she was doing when they first met. It might not even be possible to charm her again after being exposed to it once, either, so it's quite risky.