Depending on the disease and his pain level, you might want to consider letting him go peacefully at home.

My dog Sheba is 14, and has kidney failure. She has less than 25% function left in her kidneys. I put her on a special home cooked diet as soon as she was diagnosed a couple of weeks ago. She appears to be stabilized, overall. I am not going to put her down. With the kidney failure, she will go fairly quickly when her kidneys totally fail. She will lose consciousness, and just pass away. There is a chance she will just die in her sleep.

Prayers said for you and your fur baby.