Bone cancer can be highly aggressive and usually comes back in the end. I’ve read other articles that give roughly the same 6 month time expectancy.

If amputation is performed the average survival time is increased to six and a half months with 2% of patients alive after two years.

Source: https://vetspecialists.co.uk/fact-sheets-post/canine-osteosarcoma-fact-sheet/

So basically- keep in mind it’s a 10 year old senior dog. The amputation, while dogs can be resilient and bounce back from things quicker, is still going to be painful. And it’s not usually going to give him that much longer and some of the time he’ll be spent recovering. For an older dog that you may not want to put through such an extensive surgery, think about pain management options instead. Even if he’s only around for another couple months, at least he didn’t spend them in pain. Definitely talk to your vet and see what he/she thinks and if they think it’s going to give him a lot of time, or something measly like 6 months to where it might not be fair to put him through at his age.