The results of Boris' urine and bloodtests told us that he has severe kidney problems. They aren't working hard enough, but it's not clear why and how it will go from here. It can be an inflammation of the kidneys, or a tumor, or they can be slowly shutting down, etc. Drugs and/or a diet are useless at this point, because the vet doesn't know the cause of the kidney problems.
The best option now is making an ultrasound of Boris' kidneys. We got the telephone number of a specialised radiology clinic. We called immediately, but they are on vacation until August 10th. We don't want to wait that long! Then I remembered the vets of the pet shelter have equipment to make ultrasounds. We have an appointment for this coming Monday! We also have the number of other specialist-radiologists in Amsterdam, but they couldn't be reached anymore today. So we 'll see about that after the weekend.
I still hope it's something fairly harmless that can be treated. I know there are other possibilities, but I don't want to be doom-mongering. Boris isn't getting worse at the moment. We go out with him more often to pee, he eats and drinks good... *fingers crossed again*