Hi @obie and people, welcome. I'm sorry to hear that your pack is facing such a tough decision. It's hard enough when a dog is younger but add age into it and well, it's very, very difficult.

In these cases, I always recommend another opinion, even if it's from another surgeon but at a different practice so you get a more unbiased viewpoint. As bad as those tumors might appear to an untrained eye, a vet might say something different because they have the knowledge to base those opinions on. The Colorado State University Argus Institute specializes in helping pet parents make tough medical decision. Their counselors can also review his case even if you don't live in the area. 

When it comes down to it though, only you know your dog best, and only you can make that call. Yes, 12 is at the tail end of average life expectancy. But try not to get too hung up on that number. The best surgeons have told us that neither size nor age should impact the decision because every dog is different. How mobile is he otherwise? Is he young at heart? Has he recovered from major surgery before and if so, how was he during that time?