If it was the vet's best educated guess that the treatment would not outweigh the outcome (as in, a long happy life), then without a second thought I would put him down.

However, having never been in that situation before I can't honestly say what I would do. I would certainly do everything I possibly could and explore all possible options before euthanizing; but I guess I just can't say for sure until I've been in that situation. It's easy to say not that of course I wouldn't let the animal suffer any longer, but unless I'm staring at my own animal in that situation, I really can't say what decision I would make.

I will say, however, that I wouldn't give up so easily.