This same exact thing happened to my first cat. His kidneys were under-developed, and they eventually were failing. He was drinking a lot of water, and he eventually lost a lot of weight. The doctor gave him a shot to keep him alive for a little bit longer. We decided to let him die a natural death, because he was traumatized about shots. It was what's best for him... and what he wanted.
Terrible, terrible times. Just have faith that what you did was right. Sometimes you have guilt that you let him die, but I think that all cat lovers always have the right intentions on their cat. I'm sure he's in peace now.
I'm so sorry to hear about your cat. I had to put my cat to sleep on Nov 29th because of kidney failure. (I have two more cats). My cat, Miss Kitty, was also a hefty cat of 15 lbs. and she was down to 5 lbs. It was a hard thing to do, but I didn't want her to suffer. Good luck to you. Letting go of your cat is also loving them and knowing when to let go so they don't suffer.
RIP Jojo. Tough as it is, you did the right thing. Now please be gentle with yourself. That wonderful pal of yours wouldn't want you being miserable over him -- he knows you miss him, and he's waiting for you at the Rainbow Bridge.