Is this an actual breed of cat?

Six toes? That's unusual, but it's not part of a specific breed, just a type of birth defect (but not a problem). Most cats have four finger-like toes on the end of their paws and then a thumb-like one further up on their leg. If he has long fur then he's probably at least partly a Himalayan. Otherwise he's probably partly Siamese. You could have a purebred though; I work in a shelter and they're not uncommon.