I had to put down our 13 1/2 year old Golden Retriever in March of this year. He too was diagnosed with a cancerous (mass in his leg), which quickly spread into his abdomen. I took Cody to three specialists, but even with amputaion, the prognosis was grim. I decided to let nature take its course and keep him as happy as possible. He lasted three months after his original diagnosis.
I made everyday as special as I could, and made sure the kids gave him a lot of extra love. Cody was the best and I know what you are going through. Once your dog stops eating, then he is beginning to give up. Putting down a dog is very hard, but the process is painless and peaceful. I really miss my dog, but we do have plans for a new addition (another Golden) this fall.
Sorry to hear of your situation... god bless.