My beautiful Grey "Alto" has sarcoma-When to let go?

Please help! I would like advice on when to put my wonderful greyhound to sleep. He was diagnosed with aggressive sarcoma christmas eve., the tumor has tripled its size in 7-days, larger than half a football size. All in a couple of weeks. Surgery and chemo is an alternative but recovery is not good for this condition. He weighed 82 lbs. last week and has lost appx. 7-9 lbs. He is frail, lethargic and has trouble getting up. He has a good appetite and has the most beautiful and soulful eyes you have ever seen. It is breaking my wife's and my heart to see a magnificent animal deteriorate so fast. He is 8 1/2 yrs. old and is my best friend and I don't want him to suffer but I cannot think of putting him down. What is a heartbroken person to do?