In my opinion, at 8 weeks potty training is not the most important thing. Most breeders don't allow Yorkies to leave until they are 12 weeks old. Right now it would be best to focus on keeping any sort of stress out of his life. (stress can be too much playing or handling) Keep him warm, comfortable and don't let him get over tired. Have nutracal in the house. I would give that to him 3 times a day until you see he is eating well. Then keep it handy if you see him start to shake.
Now, about the potty training. An expen is a great investment. It will be your pup's "bedroom" and safe area when you can't watch him. Put a bed, water, food and a potty pad in there. If you think you may eventually crate train him, you can use the crate as his "bed" for now, but take the door off. He is too young to be crated for any length of time. These tiny dogs need food and water available.
When you catch him going potty on the pad, praise him and give him a treat. If it is warm outside take him out there and if you see him potty, praise him and give him a treat.
And if you see him potty when you let him run free in the house, don't make a big deal out of it. tell him no, and put him on his potty pad and say nice things about going good potty here.
Good luck to you and your new baby.
http://www.shooterdog.com/alexfaq2.htm