I'm so sorry you had to go through this.
And, I'm very, very sorry for your loss.
One of our cats is close to the end right now, and it's as hard as losing a person - or harder, because everybody understands if you lose a person - only those who've been there before when you lose a pet.
But my very first cat, Felix, died in a similar way, when I was 16 and he was 10.
He became ill very suddenly one day in summer. There was just me in the house, because my Nana was in hospital at the time. He wasn't pining for her, but he wouldn't eat one day, so I made a vet appointment - the earliest I could get was two days later. With us it had to be PDSA, because there was no other option financially.
But he got weaker and weaker as the day went on. I nursed him, and helped him to drink, etc, when he needed to.
But on the afternoon of the second day he went through much the same as your Bob. I assume it was his body shutting down, or his organs failing, though I don't know for sure. But it was so hard to watch.
Bless him, in under 48 hours he'd gone from a normal, happy cat, to being so weak he couldn't raise his head to take a drink.
I've clung onto the throught that at least I was hugging him, and comforting him as he went, so he wouldn't have been scared.
You cling to the same thing. He knew he was safe and that you love him.