Like most women know when they are in labor (I've been watching too many "I didn't Know I was Pregnant" episodes) you will know when it's time. If the realization doesn't come from you, you will always have questions.
When you look at him, eye to eye do you think he is a happy dog? We always tell people to examine the quality of life. Like was stated before, he doesn't know he has the c-word so his actions and reactions are based on how he feels. He is an aged dog and to be honest, I just saw this thread for the first time, he's done quite well well to still be in such good shape this many months later.
I hope you find some peace in whatever decision you make. It's never easy losing a beloved pet.