Yorkies are either very easy to Potty Train or very hard.......but it sounds like you need to feed him a high quality dogfood like Royal Canin, that is made especially for Yorkies? Give him scheduled feedings @ certain times of the day and evening and crate him too! Totally regulate his whole Life it sounds kind of ridiculous until you control his bowel movements and then once you get him used to your Routine you can actually trust him? Yorkies are very bright and smart, once they understand and it is clear what you want and expect from him he will figure it out and do everything you ask of him!
Have you bonded with him yet? I have 3, 1 male and 2 females. My male was potty trained in 2 days but my 2 females that are 3 yrs. old and her 1 yr. old puppy still have accidents but most of the time it is my fault or my BF's for not taking them out in time (more than 8 hrs. at a time). But for the record mine don't go pooh more than twice a day so it seems that he is going pooh more than average? What kind of Dogfood do you feed him? Don't give up, there is a simple answer, Good Luck!