I am so sorry to hear this. From your description, it sounds as though the leg is very frail. However, I'm a little confused about the situation. Have they determined whether the cancer has metastacized ? I wouldn't expect amputation to be suggested if it had... but then if they are suggesting amputation, why is the time line they are mentioning so short ?
(I have a friend who eventually lost her Aussie to osteosarcoma, which was diagnosed at age 2 in his cervical spine. He had surgery and received radiation treatments which proved very successful and prolonged his life another 8 years. However, the cancer came back in the same place at age 10 and was much more aggressive. He was put on tramadol and received some chemo and treatments of pamidronate to help rebuild bone. Initially they saw some improvement in his limping, but three months later they determined that things were just too painful for the dog to continue.)
Hugs to you.