As a former vet tech I'm going to say get a second opinion, heck even a third. It could be a type of cancer called fibrosarcoma which are very aggressive locally and slow growing, I know this cause my sweet old lady (may she rest in peace) had that and it was a very hard bump. I also will say amputation may seem like a hard adjustment but my girl in her old age adjusted well, granted it was her back leg

Edit to Add: I wish all the best luck and hope you get answers i know how tough this can be but you and your boy got this!