[Part of him wants to scoff at the doubt in the face of what is evident, but - he can relate, so he can have some empathy.]

I didn't believe in it either.

[It would be a little much to say he believes in it now, but he can observe that it does exist, as knives exist whether or not one believes they can be cut.]

Your lack of familiarity with magic, for one. It's common here. Not all can wield it, but you'd have to be excessively oblivious to go a long while without recognizing it.