I am very sorry to hear this.
A few tips about a cancer diet: dogs with cancer usually have a problem with their fat digestion (which is why the can get so skinny). Adding good quality omega3 oil (flaxseed, fishoil, olive oil, canola oil) is very important. Have you heard about the Budwig diet? Essentially you mix flaxseed oil with cottage cheese and feed that. It's a very good way to feed the omega 3 oil. =) Apart from that the dog shouldn't have carbohydrates or starchy foods. Only meat and a few veggies like lettuce, brokkoli, carrots or fruits (not too much fruits as they contain a lot of sugar) plus 2 serves of liver per week.
I would also supplement antioxidants, selenium, vitamin E, enzymes and cordyceps. You wrote that this treatment you want to use doesn't allow herbal remedies, otherwise I would have suggested Essiac tea as well.
To get your dog to drink more you could make some chicken broth and put some in with his drinking water. Most dogs love it and will drink it happily.
And I personally would try classical homoeopathy. My own dog has been diagnosed with a cancerous tumor in his face 2 years ago and has been treated with homoeopathy. The tumor has hardly grown since then and hasn't formed any metastases. She is nearly 15 years now. =)
Good luck!