I know you really want a puppy, but I agree with most. Give your present furbaby the attention in his elder years because the puppy is the one that's going to need undevided attention. A puppy will also probably drive him bonkers.
Is your heart set on a baby? What about adopting an older Yorkie that is not going to want to play with your senior furbaby all the time, just give him some love'n?
