Most cats are "just" domestic shorthairs/longhairs; they don't commonly have an identifiable breed like most mixed-breed dogs do. Purebred cats are comparatively rare and aren't usually allowed to breed freely. The only way to truly tell if a cat is purebred is if you have his registration papers.
That being said, purebred Russian Blues always have a particular shade of green eyes, and his are more yellow. So I can say with a fair level of certainty that he isn't a purebred Russian Blue, but other than that, who knows? He certainly is a handsome fella, though.