Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I am so sorry to hear that Coors has been diagnosed with colon cancer.
Without knowing what type of cancer it is it is very difficult to know how long he has because different cancers will grow at a different rate, and some readily metastasize (spread to other locations) which will create other difficulties for him, depending upon where the cancer spreads.
The two most common types of colon cancer in dogs are lymphoma and adenocarcinoma. I suspect that your fellow has adenocarcinoma because he has many palpable tumors which is a more typical presentation for adenocarcinoma, whereas lymphoma tends to be an infiltrative cancer, with discrete tumors less likely as a presentation.
Adenocarcinoma is a cancer that likes to metastasize to the local lymph nodes in the abdomen as well as the liver. This can cause blockage of the colon if the lymph nodes get too big and press on the colon. Spread to the liver causes poor liver function and that can decrease clotting ability.
Prognosis in dogs with multiple tumors that aren't surgically resectable (like your fellow) is weeks to 3 to 4 months. Chemotherapy usually isn't done because these tumors are not responsive to chemotherapy.
Things that can be done for him to keep him comfortable are feeding a canned low residue diet like Hills i/d or Purina Veterinary Diets EN. These highly digestible foods produce little stool which is helpful. the less he has to pass by those tumors the better.
We can keep stools soft with Miralax added to the food, 1/4 to 1/2 teaspoon sprinkled on the food twice daily. We do need to make sure he is well hydrated using Miralax so I recommend adding low salt chicken broth or warm water to his meals. Another option for keeping stools soft is a prescription medication called lactulose. You can discuss which would be better for him with your veterinarian.
It may be helpful to put him on Piroxicam, which is an anti-inflammatory that has shown some anti-tumor effects as well. It can potentiate gastrointestinal bleeding though so you need to discuss the pros and cons of use with your veterinarian. Another option is using a different nonsteroidal anti-inflammatory such as Metacam or Deramaxx, which may not be quite as effective but is less likely to lead to further bleeding or ulceration.
You will know when it is time to let him go by his appetite and attitude.
Dogs that are no longer eating, or vomiting with every meal are signaling that it is time to consider humane euthanasia.
Dogs that are no longer interacting with the family, but rather going off on their own, are letting you know that they are too painful or sick to enjoy life.
If he is no longer able to enjoy doing things that he likes such as walks or playing ball, although we know that it will be for less time as he feels weaker, he is telling you that he is no longer enjoying life.
I know that you love him very much so I think that you will be able to read his subtle clues that he is no longer enjoying life, and that is when it is time to let him go gracefully. I am so sorry that you and Coors are forced to go through this.
Please let me know if you have any further questions after reading my response.