We lost our first dog, Gibby, a littler over a year ago to cancer. He was only a couple months away from turning 5. Despite the short time we had with him, he was still such a huge blessing to us and lived way longer than we ever though possible. We found out he had cancer when he was a little over three years old, but he pushed on, with a great quality of life, for a long time. I remember my last words to him were "see you later," as I was heading to work, but while I was away his health started quickly declining and my parents knew it was time. I wanted to be there for him, but perhaps it was best that I didn't have to see him like that.
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Like everybody else here has said, just take as good care of him as you can until you know it's time.