Help my yorkie wont eat his food.?

So just adopted a yorkie puppy. He is about 7 months old. He was given Eukenuba Puppy food but he never ate it. Our maltese boy eats blue buffalo. He was eating that for a little while and now both have stopped eating the food altogether. We started to shop for new Holistic food and someone suggested wellness to us. We started to ween off the blue buffalo but neither one has eaten the food. We tried buying wet food to open up their appetites but still nothing. I am worried since my yorkie is a pup and very active. We do give him a high calorie nutritional supplement to prevent the hypoglycemia and he has training treats throughout the day. (He is in training) Is that enough??? I think its not. Also If anyone has any ideas or Healthy tasty food I can give them please let me know. We try to stay away from by-products, fillers, grains, and dyes because we don't want tear stains on either one of my little guys. Thanks for the help everyone....