I am so sorry to hear of your little guys cancer
have you looked into any holistic treatments at all? when i was first researching raw diets for my guy, i came across many stories of people with dogd with cancer that were helped by putting them on raw diets. I can't remember at this point how far along the cancers were, etc, but it might be something to look into. sometimes cancer thrives the more and more chemicals we put into the body. i speak from my own experience as i am a cancer survivor...it wasn't until i stopped taking medications and started eatting a strict, non-processed, organic, mostly raw diet that i went into remission!
also, a holistic vet might be able to work out a diet plan with suppliments for you. i know with humand, high doses of vitaminC have been known to work. please do not try this with your guy without talking to a knowledgable vet first though, as i am not a vet...just trying to brainstorm some ideas for you.
good luck and kisses to your little guy.