I think a lot of it is instinct in knowing when to let go. We had to put my mom's cat down last March (we'd only had her since December of the previous year) due to a huge tumour growing on her side. But I think once they stop eating/drinking and enjoying the things they used to love, then it's time to say goodbye
I'm so sorry you're going through this, he's beautiful. I'm not looking forward to the day I have to say goodbye to my little guys
*many hugs for you and your family quasi*