I am so sorry. It is so hard to see our babies age and become so ill....not knowing for sure what to do. I was just there recently and relied heavily on the opinion of my vet. Thankfully, she is not one who will continue to treat without letting me know what the chances are of a full recovery.
It is a hard decision to let a pup go but I always want to be able to do that for them.. not for me because I want them to be with me forever.
I wish there was something to say to make it better. It sucks terribly to love them knowing they won't be with us as long as we would like. All I can say is, enjoy him for the time you have him...a day at a time....and listen to clues. I believe they let us know when they cannot go on.
My best to you.