Your post breaks my heart -- probably because it hits so close to home for me.
Have you tried making him more comfortable with some doggy NSAID-type medication? We did that with my old German shepherd. I *think* the doc prescribed rimadyl or rymadil. It's been more than a decade so I'm not sure what it was.
With my old shepherd, he went through a very short period where he quickly lost a ton of weight. His hip bones showed. And then one morning he was paralyzed or so weak that he could not move his legs. We rushed him to the vet and were told that it was old age causing his organs to shut down. That is when we made the decision to let him go. I am completely at peace with that decision.
However, I am continually haunted by the decision we made to put down my last shepherd-mix. He had epilepsy. Once we got him on a dose of medication that controlled his seizures, he was fine for many years. Then, when he was about 10, he got a tumor in his mouth and would not eat. We took him to the vet who wanted us to bring him back at a later date to surgically remove the tumor. It was going to be costly. We had decided that we'd have the initial surgery done but if the tumor was malignant, we'd have him put down. When we got home, he had the absolute worst seizure and it seemed to go on forever. I thought I was watching him die a horrible death. I freaked. I had my son immediately loaded him back in the car and we took him back to the vet. I chose to have him euthanized that afternoon. Looking back, it was a knee-jerk reaction caused by my fear of watching him die during a seizure coupled with the cost of the future surgery that convinced me to have him put down. To this day, I wonder if the tumor was benign and if he'd stop seizing once he had the tumor removed. I can't help but think that if I'd not been so worried about what might happen that I could have had another couple of years with him.
This probably has not helped you at all. If I had any advice to offer, it would be to think it through carefully since it is a decision that you will never forget. If you truly feel it is his time, you will likely be at peace with the choice to say good-bye. If you steel yourself for euthanasia thinking it will save you from watching him die sometime in the future or if you think it will save you a thousand dollars (in my case both), it will probably leave you unsettled for some time to come.