I am not a vet so take what i say with a grain of salt. My 9 year old 135 lb Saint Bernard was diagnosed in June.  She had her right front limb amputated about a week after diagnosis. She had her first dose of chemo the day after amputation.  She’s now finishing up with immunotherapy.  She’s doing GREAT on three legs.  It was a rough two weeks immediately afterwards, but they adapt.  While I’d be depressed for years if i lost a limb, she’s completely over it and back to doing most things that she did before amputation. i just want to give you a little background. 

My advice based on my experience: ONE, take her to a specialist.  Meaning a vet hospital that has an oncology unit.  They will usually have her evaluated by a onco/ortho surgeon to assess whether or not they are good candidates for amputation.  TWO, decide and act quickly. This is an aggressive cancer. 90% of dogs have at least micro-metastasis at the time of diagnosis (cancer cells that have spread throughout the body that you can’t see on an x-ray or CT). That is why they recommend chemotherapy even after amputation of the tumor.  THREE look into some newer treatments that are out there. Immunotherapy is a really big one right now.  There is also a treatment out of the vet research hospital at Colorado State University that involves a high dose of Losartan (usually used for cardiac conditions) and an oral chemo drug that i can’t think the name of right now. This treatment is used at the point where there is visible metastasis.  Over 50% of dogs have responded to it in studies and it’s worked to extend their lives some. FOUR, i would also recommend looking into a company called FIDOCURE. It’s a “targeted therapy” treatment company.  I’m currently having them analyze tissue from my dogs cancer to see what recommendations that they have. 

FIVE, if you do wait until the end of the month to do amputation, consider taking him to a canine physical therapist for the next couple of weeks to work on his core strength and cardio health.  I know that it’s only a shot amount of time, but i would say that anything helps.  Ophelia (my dog) benefited GREATLY from having been in PT for 18 months prior to being dignaossd with osteosarcoma.  I will always always credit that with having helped her transition to three legs immensely. BTW, Ophelia was in PT because she has mild arthritis.  

SIX, look into adequin of your dog isn’t already on it for the arthritis.  It helped another dog of mine a lot.  It is almost a miracle drug with what it does.  

Don’t give up.  All you can do is your best.  

good luck.