Rikki, the world's toughest cat

I'm worried about my kitty, Rikki. He is a purebred Sealpoint Siamese and he's about 10.5 years old. About a month ago he started randomly pooping outside the litter box. I figured he either hated the litter or I needed to scoop more often. I changed back to the litter he was used to and made sure to scoop at least twice a day.
I took him to the vet about a week and a half ago. She figured he was constipated and gave me some meds and put him on a special diet. I've been feeding him that and he started pooping in the litterbox again. One thing I noticed was his water fountain was drying out quicker and quicker. I have to refill the reservoir almost every day instead of every three or so days. He has been pissing like an elephant too. He started pooping outside of the litterbox night before last. I'm really afraid he might have kidney failure because it's common in cats especially Siamese which start showing signs around the age of 10.
Rikki has been the only constant in my life in the last three states I've lived in. My guy friends are afraid of him (he doesn't like men) and my gal friends say he's a little shit. Rikki is a picky cat. I know he's an asshole at times even to me. One thing I know is I would be completely crushed if I had to put him down. Wait, let me say that again. I will be a mess if I have to put him down. I don't want my kitty to die yet. I don't want him to be in pain. I don't want him to live out where something as simple as pooping makes him hurt.
I have to take him back to the vet on the 25th of October. I have to give him kitty valium an hour before I take him in so that he doesn't tear up the world when they're checking him out. I don't want to walk in to the vet's office with a scared kitty and walk out with an empty cat carrier. I am petrified to lose the only thing that is truly keeping me sane right about now.
My baby is not ready. I am not ready. We are not ready to be apart from each other. If there is or was ever a God... can you do something now because I'm about to lose all faith that I have left. :(