Gee whiz, Wolf! I HATE to hear this kind of news. As to advice, I really can't give any. I think you, being on the scene and loving this great dog the way you do, are the only one who can or ought to make the decisions in a case like this. All I can say is God bless you both and may God lead you to the best and most "right" decision. The only bad thing about great dogs, is the loss we feel when we lose them, or when they're hurting. Let the Lord lead you, and if it's his time to go, just let him go.
Most theology says that dogs have no soul, and can't go to Heaven with us. I'm not so sure of that. No animal in creation loves the way our good dogs do, and I can't help but wonder, and tend to believe, that this may well earn them a place in Paradise. It'll be great to see all my friends and relatives who've gone on before me, especially Mom and Dad again. But how could anyone NOT be overjoyed to see our best old canine friends, running and jumping on us, so happy to see us? If I'm wrong, I guess I'll just have to be wrong. I trust God to make the best of all possible decisions much moreso than I do myself. But whatever the case, I know for sure that the love they instill in us, and give us so selflessly, will never die. It can't help but change us, and make us better people. So in any case, at least part of them will always be with us, no matter what. I find some comfort in that, even when I mourn the loss of "one of the good ones." God bless you both.