This morning we called the veterinary specialists clinic in Amsterdam for Boris. They could squeeze us in to make an ultrasound of his abdomen: kidneys, bladder, etc. Without any problems Boris let us put him in the awkward position on his back. Ron held his head / front and I was at the back, but we didn't really have to hold him tightly. His belly was shaved, water and jelly applied, but he didn't utter a sound.
The radiologist showed us what he was doing and explained what we saw. The kidneys looked good, structure and size okay. In one of them was a little cyste, but it was so small that couldn't have caused Boris' symptoms. His bladder showed an inflammation, he has cystitis. His prostate was enlarged. These things together explain all the symptoms, including the aggression towards the dogs he attacked.
With the results we went to our vet. He chemically castrated Boris to bring the prostate back to normal proportions. He got an antibiotics shot and pills against the cystitis. To gain the weight he lost we have permission to spoil him rotten. :) His food is very good, but he needs more fat and a little extra to come back to his normal weight. In three weeks his urine and perhaps blood will be tested again to see how he's doing. His hotspot is almost healed.
Of course the radiologist and vet can't look into the future, but there's no immediate cause for worry, nothing life-threathening. Yay!!