re: Old dog. At what point you say good bye?

I agree. Old age isn't usually a reason dogs get real skinny, unless he's REALLY old.

It's possible he has some ailment that can be treated.

As for your initial question, your dog will let you know. He'll stop eating, and he'll show little interest in things that would usually excite him. That's when it's time.

I started talking to my vet about it close to a year before I put my first dog down about 3 and a half years ago. He kept telling me that she'd let me know, and one day she did.

Just enjoy the time you have and know that you've been a great 'dad' to him.