The thing is, some of the older SF reads a little dated - it may not appeal in the same way to newer generations, even if the ideas remain imaginative and compelling, and hooking a beginner into the genre might require something that appeals more to modern tastes. For instance, Andre Norton wrote some very imaginative tales, but reading them now, the style feels a bit stiff and the characters a little stereotyped. Doesn't mean they're bad, of course, and many elements would likely entertain even today, but expectations and tastes aren't the same these days, and younger readers are more likely to pick up on stuff that was glossed over or just not seen Back In The Day (some of the racism and sexism and such.) It might take developing a taste for the genre first to get him to look past such things (or understand that things were "different" then), to let him enjoy the classics. JMHO, of course.
On the graphic novel front, IIRC there are some GN adaptations of classics that might be worth a look, maybe as a way to get him interested in stories whose style or density might not initially appeal to him. (Then it's "if you liked that, read this..." your way into spreading the SF bug to a new generation.)
