Well, if Smallville is anything to go by, he seemed quite vulnerable to Lana's powers when she was taken over by the witch. In the L&C series, he was vulnerable to hypnosis, which isn't exactly magic, but it's associated with it.

I'm sure he'd have more resistance to magic than normal folks, but I don't think he would be completely resistant.

I believe there's a hero in all of us that keeps us honest, gives us strength, makes us noble, and finally allows us to die with pride, even though sometimes we have to be steady and give up the thing we want the most. Even our dreams. -- Aunt May, Spider-Man 2