I commend you for rescuing Buddy. I'm also sorry to hear that he is not feeling well. We just went through a similar experience with our 11-year-old Golden, River. He developed cancer and our vet didn't recommend treatment of any kind due to the aggressive form of cancer he had.
Your question is one that is dear to my heart. Honestly, I feel that during times like you're experiencing we often wait too long in making the best decision for our beloved pets. I think the question you have to ask yourself is whether or not Buddy has the quality of life that is best for him. If there isn't anything you can do to help him genuinely recover and get better, well, I know it's hard, but I agree with Nolefan. I think you already know the decision you need to make in doing the right thing by Buddy.
I just went through this a couple months ago. River was the greatest dog anyone could have ever wanted. He was truly a gentle giant that hung on every word I said, or at least he made me feel he did.
Once he stopped eating and became lethargic, I knew then that he was telling me it was his time to say goodbye. Making this very hard decision is never easy, and Lord knows we have had to make this decision more than we would have liked. This tough decision is all part of having pets. They never live as long as we would like, and often they leave us way too soon, but in the end, your thoughts and heart need to be focused on what is best for Buddy.
The fact that he is not connecting with you and that he is doing his business in the house are signs that he is having trouble with the life he has. You mentioned that doing his business in the house is so unlike him. The ole Buddy you have known for years would not be happy doing this. I'm so sorry!
Focus on all the wonderful memories you have had with Buddy. Remeber the good times. This is what Buddy would want you to do. God Bless!