I am sorry to hear that your dog has been diagnosed with cancer. Derma-IonX would be safe to give your dog along with any other medications or supplements that you may give. It should not hasten the cancer. It's in a micronutrient base and is homeopathic in nature, helping to cleanse the cells, reduce inflammation, and stimulate healing. You need to reduce inflammation in the body to make him more comfortable.
Other things that are helpful for allergies and reducing the resulting inflammation include avoiding as many allergens as possible. You might consider a serum test to check for possible allergens. Sometimes certain grains can be a problem. I find grain-free diets, raw food, a high quality fish oil, probiotics, and digestive enzymes really make a significant difference in the quality of life. What do you feed your Westie, including treats? You do not want to add fuel to the cancer fire. It is important to strengthen the immune system and the above mentioned suggestions would be a place to start. Please let us know how you progress and if we can answer any more questions for you.