It's a tough situation, I know. You need to evaluate if he is suffering and how much, and go from there. Sometimes loving them means recognizing it's time to spare them from an agonizing death. If, however, he's just slowing down in his old age but not showing many signs of pain and suffering, then he should definitely be allowed to live his life out as best he can.
We had a two year old black German Shepherd named Buddy who developed cancer in his esophagus. The decision came down to euthanizing him peacefully, or letting him starve to death. It was hard, especially because he was so young and we had rescued him from a hard situation and life was just getting good for him, and I still miss that goofy dog, but letting him die painlessly was the right decision to make.
I'm not telling you that story to influence your decision one way or the other. I guess I'm just trying to encourage you and sympathize with you somehow. I'll continue to lift you up in prayer for wisdom in this matter.