dna tests are not expensive nowadays. What kind of papers did the breeder give you? what registry? If it was anything other than akc, then i would think the breeder duped you. Akc, while not perfect, requires better record keeping, while other registries such as aca, apri, etc does not. Before I knew any better, I registered Uni with aca. They do not even require a picture, or a witness!
I could have registered a horse and a yorkie. seriously.
Also if he does turn out to be a mix, you need to get at least some of your money back. While $500 is not a lot to pay for a purebred , it is too much to pay for a mixed breed. Shelters are full of mixed breed dogs that you can adopt for $200 or even less sometimes. I would not want to support someone who purposely breeds mixed breeds, esp one who then lies and sells them as purbred.