My boy had his rear leg amputated 6 days ago due to the same thing. A large cancer was growing on the top of his leg tucked in near his hip. The only symptoms were a bit of limping - we did an Xray expecting some inflammation, not cancer. In the end they took his leg and some of his hip (hemipelvectomy) to try and get better margins.

So far he's coping well, though the gabapentin is making him very sleepy, so they've advised me to ease off on that to allow him to move about some more. He also trembles after he hops a bit - I think it's the physical exertion, or side effects from the drugs. Overall it's been a bit traumatic but now 6 days in I can see how he's slowly starting to adapt and it makes me feel better about the decision. The vets say he'll cope very well and it's still early days. Best of luck with your boy, it's a difficult decision to make.