He's back at the vet and going under sedation for treatment of a urinary blockage. He will probably be at the vet's for several days, but they expect that he will be fine. They are going to look for kidney stones et al. and let us know what they find out.
Thanks for the well-wishes all! They're much appreciated. The situation is not trivial, but at the moment at least looks like things will turn out all right in the end.