GAH, I am so upset with my vet! I just read up on how plaque builds up under the gumline and then it destroys the connective tissue holding the tooth in place. They always said, oh his teeth look okay, given he's a yorkie... but he has lots of plaque on his gumline. GAH! I've worked so hard to keep his teeth in good shape, and now it's obvious he should have been getting yearly dentals.
I am travelling this month, so I have him scheduled for an emergency consult today at a vet recommended by a friend. One of his other teeth is loose. I just hope the damage can be undone.