IF I were you (and I'm not
) I would wait until you get your yorkie before I ordered adult-size clothing. A breeder can guess the size and that's all it usually is - a guess. You'll have plenty of time to contemplate what to buy for him as an adult. That way you won't have clothes that he might either never grow into or might quickly grow out of.
You can get a vest harness which closes with velcro and they grow with them for awhile.