I would seriously think about referencing the old Forsaker PrC (Masters of the Wild, pg.57) for a little insight, it uses DR and fast healing as well as SR along a progression.

Interesting class. Hmm, constructive.. Why they got enhancement bonus if they are unbelieving in magic. I got the meta reason but not in-world reason why disbelieving magic makes your equipment stronger.
I'd say it's merely his unshakeable belief in the mundane, and his own mastery of that belief.