Oh Lou
. Ivan sounds so dear. What a blessed life you've given this little baby, he is so lucky to have you, and you him. I don't know the answer to your question...some people say you "just know". I had to put down my 20 yr old cat a few years ago, she had kidney disease for a long time...and then one day, she just went into what the vet thought was failure, and we had to put her down. That's my only experience with putting a pet to sleep.
All I can really hope for you and Ivan is that your heart will tell you and his heart will you when it's time. I wish I could give you a huge hug right now. It's so heartbreaking to let go.