I've been down this road too many times to mention as most of the dogs here were bred by me, lived their entire lives with me and then were sent to the rainbow bridge to wait for me.
I get through it by realizing that I know how much love and caring they got their ENTIRE lives. That's something that a lot of dogs out there don't get.
It doesn't make the diagnosis or prognosis any less painful but maybe it will help you because you know what I know - the quality of the live he has had and the love you will give him by giving him the gift we give at the end.
I hope you get many days of his presence before he too leaves to wait for you.
Please tell him that we are all sending comfort to him and his people.