I know what you are going through, and I truly feel for you. My biggest suggestion would be to try not to let your dog see you upset, I think they pick up on it and it frightens them, which he doesn't need right now. Spend as much time with him as you can, and enjoy every minute.
I really feel for you and what you are going through right now.
Is it in his spleen? Or heart? Or both?
I know how wonderful it is to know and love a dog and to be loved in return. They are indeed more than "just dogs"....well they are to me anyway. Mine is like my little sister. Actually I can't find the words for it -the bond I feel with her. She is a character, just as a human would be, and she is true, and good, and a very fine friend.
So I know that one day I will have to rescue her from whatever sickness she will have. I will have to make that ultimate decision, to protect her against further suffering by doing the unthinkable....ending her life.
I am so sorry.
All I can say is you are very welcome here, and there are others who have been through this. I am sure they will be here for you.