Another one who has to say goodbye ?

My gorgeous old cat is 16. He is the soppiest, most placid cat who has spent his life just wanting cuddles.
A few years ago he was extremely unsteady on his back legs, but on Metacam and time he improved.
A fortnight ago I noticed him struggling again. But once he’d had a stretch and got going, he was ok so I put it down to old age arthritis.
Last week he deteriorated. His back legs were so weak and wobbly, as if he’d a couple of bottles of vodka.
Vet suspected a spinal tumour but wasn’t sure and thought Metacam would relieve swelling and show a significant improvement.
But he’s got worse. Much worse. His back legs are like a huge burden to him and today he’s given up even trying to use them.
I have shut him in my room with a blanket, food, water and litter tray for the last couple of days as I feared he would fall from the top of the stairs.
Earlier this week he was still clawing his way onto my bed (literally dragging his hind legs behind him).
Today he hasn’t even bothered attempting that.
I have another vets appointment at 6pm and I have prepared myself to say goodbye but I feel so sad.
He has been the most adorable companion and I will miss him so much
Obviously I don’t want to say goodbye, but I feel it’s the right thing to do. But part of me is worried the vet might suggest scans, investigations etc and I will then feel so guilty saying I don’t want that. Do you think they will respect the fact that I just want him out of his misery now?
There is no way I would put him through operations/chemotherapy etc so I feel investigations would all be in vain.