Sick cat should be euthanized?

Please advise what could be causing abdominal retractions, weight loss & weakness in a 16 yo cat. Could it be cancer? His vet wants a work up. I think a work up may put him thru more misery. He is not eating by himself, I have to feed cat food by spoon& water with dropper. This is getting worse for last two weeks. Is it time to be euthanized ? It is really painful to see him suffer. He does not appear to be in pain & does not cry but you can see the abdomen move in and out with so much force that it is a wonder he is still hanging on. Please say what you would do. This is an amazing kitty, loving & gentle, & an avid mouse hunter. We love him so much.