I'm afraid I'm with those who say that it is time to make that very hard decision. I once put off for a week having a cat put to sleep out of pure selfishness - I was acting in a play and didn't feel that I could cope with going on stage having done the dreadful deed - and knew deep down that I should have done it earlier although my beloved cat was still able to move around on her own but was very ill. A couple of years ago I took my 15 year old terrier to the vet the day she became unable to walk, and I knew it was the right thing to do. It's a horrible thing to have to do, but it's far worse to let the animal suffer. Vets, in my experience, don't come straight out with it and tell the owner when it is time even if they might hint at it. Whether this if for the money or because they don't want the owner blaming them I am in no position to judge. But I know when I saw my mother suffering, with the cancer having got to her brain so she didn't know who I was, I really wished I could do for her what I could do for a terminally ill animal.
It is the kindest thing you can do, to stop prolonging the agony. It's the hardest thing, I know, but you are responsible for your dog and if you really love him you know that he will not get any better and if he's losing weight and getting worse then he has little or no quality of life. I really feel for you, but it's time to put your pain to one side and stop his. Thinking of you and your dog brings back memories of the cats, dogs and horses I have lost and will never forget. You won't forget him, but please don't make yourself regret leaving it longer than you should.
Best wishes from me and my latest dog. I swore I wouldn't get another one, but my friends decided I needed to move on and gave me a puppy for my birthday last year. She's sleeping here beside me and I wouldn't be without her.