Brian, sorry to hear this.

Large breed dogs do better with forelimb amputation that hindlimb.
Ask to be referred to an ACVIM oncologist (speciality board certification in Internal Medicine sub speciality: oncology) if you need a second opinion. This tumor needs to be staged and he needs to be evlaluated for spread to distant organ sites. This sounds like an osteosarcoma.... very aggressive. One thing, it is harder for an owner to accept his 3 legged companion than for an animal, they respond dramatically and do well in the majority of times with only 3 limbs.

Best wishes,

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