This post shouldn't be as scary as some of the others lol since I'm pretty sure it's a cut and dry answer. I'm not positive and I'm definatly no expert, but I have a parti (her body is mostly white) so I looked it up on the AKC website and a few other reliable sources. From what I gather, a Yorkie is "allowed" to have a
very small spot of white on the chest area (there are actual dimensions on the AKC site I believe). So, if yours has a prominent white area, as far as I know, he's considered a parti. My aunt's Yorkie (from the same litter as mine) has a white spot similar to what you're describing. I could be wrong but that's the way I understood it. *Disclaimer (LOL) - please don't anyone yell at me if I'm wrong
but please do feel free to correct me
