Had to deal with something similar 4 years ago with our lab. He had a tumor in his side pushing against his stomach that was slowly starving him to death. Surgery would probably only prolong his life a few months. We put him down after he had lost a bunch of weight and couldn't sleep or breath very well and also while he wasn't suffering to much. I held him in the back of the truck in front of the vets office when we put him down. Was very hard but was worth it for my wife and I and him. I'm glad we did it that way. He was calm and quiet. So, I feel your pain it's very hard to see and to know when. Best of luck and enjoy them while you can.