Oh, then, maybe he's just so active he's running off his calories. I don't know how much they should be gaining at that age but if your vet is not worried and you trust him, he should be okay. If you are still worried and no one here has some ideas in the next day, maybe you could get a second opinion. He sounds darling! I love the tiny, frisky Yorkie babies!!! Keep us updated if you can.
