Your dog is old and sick.
For the best results, I would stop looking for advice on the internet, forums, and the like.
A lot of misinformation out there. Not to mention the homeopathic miracle cures and such, example: dynovite. These things may add to his pain and discomfort.
Instead I would do exactly as the vet that is treating the dog instructs you to do.
Diet, medication, etc. No supplements. Just the prescription food with water added.
The food recommended for your dog is good. Prescription food is formulated for specific conditions. He may need to stay on it the rest of his life.
“Royal Canin Veterinary Diet Ultamino dry is a complete dietetic feed for dogs formulated to reduce ingredient and nutrient intolerances. Royal Canin Ultamino is guaranteed by strict safety and food control processes to avoid risk of potential protein contamination. This food requires a veterinary prescription.”
Science based veterinary medicine here http://skeptvet.com/Blog/2016/07/more-nonsense-from-holistic-vets-about-commercial-therapeutic-diets/
