Realize they're going by an AVERAGE life span, but you know individuals can live much longer than average. Cats die naturally at ~15 yrs (only counting cats that get vet care), but the oldest was like 32 years old! I don't know what would be a record for a Pom.
I would get bloodwork now and see what the general health is. If it's off, you may not want to do it. If it's all good, I would probably go for it. The vet only "thinks" it's benign. If you're going to do it, I would do it sooner rather than later when (a) he's ill from either cancer or other elderly concerns, or (b) the tumor's larger and more difficult to remove.
I don't really have strong feelings one way or the other. It's going to be a hard decision based on what's right for your whole family.
BTW, $500 sounds a little low to me. Does that include bloodwork and histopath? Then again, my last boss hiked prices severely, so I don't have a good grasp of average.