To me it looks like it might be a mix, but I agree with everyone that he is adorable either way. If he does turn out to be a mixed breed, keep in mind that mixed breeds are often healthier then pure breds.
If it matters to you, then if people ask, say YES he is 100% pure yorkie, and I can trace his family tree all the way back to England! His great-great-great-great grandmother was a ratter for the Queen! Who's going to contradict you?
