I'm so sorry to hear about Nook's fall. It's tough trying to decide if he's suffereing or still has some good days left. I have to agree with everyone else though, I think you'll know. My RB terrier made the decision himself, but I've had 2 friends have to make that decision in the past 6 months. Both of them say they knew it was time.

Give him a big Hug for me.