What is he currently on? It is always a good idea to transition slowly when changing foods. I think frequent small meals are better for everyone for the most part but it is good that you are prepared to be extra watchful of that little guys blood sugar levels while he is so young and tiny. Although the breed standard is that Yorkie's do not exceed seven pounds there do tend to be a lot of variances so it is hard to say how much your little guy will weigh. I advocate for weighing puppies and measuring food myself
that way you know for sure what your little guy is getting. Congratulations!