It's a tough situation man, especially with the unknown diagnosis, but it seems like torture for you and maybe even the dog to keep going on in the current state. It's all about the quality of life. I was in your shoes a few months back, and fortunately I knew the diagnosis so it made the decision cut and dry. I brought him home to see the wife and kids and cooked him some deer steaks for his last meal (which he ate eagerly after having to be spoon fed for several weeks). I'm not an emotional guy, but it takes a toll. I cried more that day than I did when my dad passed. It's not easy, but it's inevitable.