All dog owners have been where you are at some point in their lives. Don't let the poor guy suffer. Thank the good Lord you've had him in your family and know that the BOTL's and SOTL's here are feeling your pain. I've had to put down two very long time and loving companions within the last two years and it's heartbreaking.
Only you will know when it's the right time. When it comes just know we'll all be thinking about you. Best wishes.