He's way too young to be consistent, if he's going half the time on the pee pad, that's good. A mother can smell when the pup needs to urinate and will nudge them to the pee pad, so keeping them with their mother can be helpful in training. When Joey was out of the x-pen, my eyes were glued to him and a pee pad was never far away. They will not go far to seek out a pee pad at that age. Some dogs don't like to pee and poop on the same pad, for in nature, they don't pee and poop in the same place. Since you know he can only hold it for about 20 minutes don't allow him out of the pen for that long. Also, ignore the mistakes, and praise the success, as you are doing. If you punish for mistakes, they tend to have later problems with submissive urination or hiding or eating their poop. It's good that you keep food and water available 24/7, some people withhold it thinking that it will help with training, but experts say it doesn't help. Your puppy needs to drink water in order for his bladder to gradually stretch, and food available all the tie helps prevent hypoglycemia.
Personally I don't believe that you should expect too much until a yorkie is at least 6 months old, and than it's just 95%!
