He looks ALL Yorkie to me. His face is a little long, but some yorkies get long and lean and some are more rounded and squat. I have brothers from the same liter and EVERYONE knows one of them is yorkie fullbreed, the other one people question all the time. Obviously if they are the same liter they are both full breed yorkies, but the differences are amazing! One is LONG and LEAN and about 8.5lbs, silky silver coat, up ears... the other is rounded and shorter, darker in color, cotton coat (no silver at all) not sure if his ears are up or not (never been tipped to see) and hes only about 5lbs!
Your baby is adorable! And Im not a pro at this, but to me he looks all yorkie, those colors are pretty distinguishable! He has a bunch of time left to grow into himself!
