I only wish having dogs and cats "put down" were counted as cruelty against HSUS. A friend was having that problem last week--a lovable old dog was dribbling all over the place. I saved extra newspapers for them to put on the floor where the dog was. There's medication that helps some animals with bladder/kidney disorders, but it's not cheap, and wouldn't have helped the old dog much anyway. Too far gone. After a few days of dribbling he developed a fever and started vomiting. So the vet helped shorten his pain.
Sorry it's happening to your dog, though. Sorry their lives are so much shorter than ours.