Thank-you for the rescue suggestion! I have an appointment with a vet who works with dogs like this at one of the rescues.
Someone commented that it was a discipline problem. But up until now, the vet has never been a problem. He liked his vet and the staff up to the December visit. He went in, sat down, and happily went in a room. We were not told the nail trim was a struggle. I don't know what happened, but it definitely stuck with him. Panic set in as soon as he was in the door. Nothing we had ever seen before.
Had I known, we could have worked on it in the time ahead of the appt. And nothing came out until the second visit to that vet. Not fair to the dog, or to us.
It is going to take work to get him comfortable again, but we will work on it.