At this point it would have been easier to tell him that it was a prank at the start, I'm sure after explaining and telling him that she actually likes him now everything would be fine.
But it's manga so let's wait for the drama I guess
At the start of the manga I hated Kazuma, but I feel he has undergone some character development in the last few volumes, becoming a better person for Mizuhara and stuff.
Still dumb, but he's rocking that suit tho.