For snacks, you can try natural foods like little pieces of apple(peeled), carrots, watermelon, yogurt, etc. NO grapes or raisins though.
As far as food goes, the bag should be able to tell you how much to feed your puppy. Some of us have started homecooking since the recalls, but you can also take a look at
www.dogfoodanalysis.com Look up the food that you're feeding and see what it rates and why. 4 and 5 star foods are recommended for yorkies. Also, I would recommend free feeding your puppy especially since he's so young. Just leave a bowl of dry food out all day, and make sure he has access to it at all times so he can eat when he gets hungry. Good luck!!!