First of all, thank you to all who offered advice and condolences.
Our vet suspected LS until I described how he had deteriorated in the one day since he saw him, then the vet changed his tune immediately and referred me to a neurologist who said to bring him in immediately. Of course, in my experience, a neurologist can only give you bad news, as was the case here. My boy was examined and the news was not good. How not good could only be determined with an MRI and considering such rapid decline...
Anyhow, I did the best thing, the only thing, I could do for my best friend and I let him go.
I am grateful that once I made that decision, the neurology team fully supported me. They did not give me any false hope because they had seen this before and they knew how it would go. So, considering the blood test results which showed abnormal but not alarming kidney function, the most likely culprit was brain, kidney, and perhaps other tumors.
His passing was peaceful and he is no longer in pain, and we are coping as best as we can with our loss.