Gosh, you've just had him a few weeks haven't you? I don't know how much research you did before choosing a yorkie, I know you asked a lot of questions, but sometimes we don't ask the right questions, and important information isn't given. That why reading other people threads comes in handy, they ask questions, we didn't even think about. Yorkies are hard to housebreak, harder than most breeds. I would say don't expect 100 % until the age of 6 months, and that's if you do everything correctly. Some yorkies are easier than others, and some poop 5 minutes after eating so you just take them within 5 minutes after eating and they're trained. Others aren't so easy, and some dogs like a separate pee and pooping pad. You have to look at signs your dog gives before he poops, some circle or walk back and forth; you can place them on a pad or take them outside. Always praise the success, and give a small treat, and ignore the mistakes. This link has some good tips.
Free House Training for Puppies and Adult Dogs - No More Accidents Starting Today!