Surely, I hope your dog does not have cancer, but if he does, it is hard to say what his amount of time left might be. The vet might be able to give you an idea, but it depends, again, on whether or not he does have cancer and what clinical stage it is in.
No one really can ever give a definite answer on any animal's life span with a disease such as cancer. Perhaps your dog will be fine, and he may just be suffering from some disease that is viral that has affected his organs so severely.
I found another website that is very good about splenic masses and cancer and you might want to look at that. Hopefully your dog does not have this, but it might give you some good information. It always helps us to learn as much as we can, and then we may not feel so helpless when we have to deal with diseases with our pets. The site is:
If he is still eating well, and acting pretty normal now, with normal bowel movements and urinating okay, then I would just be thankful for each day you have with him. Just like any pet we have, we just never know when that day might arrive when we have to make the decision to let them go, if their quality of life becomes unbearable and painful for them.
If you have thought about asking another vet for an opinion, and if there is something else that might be done, you will probably end up paying as much all over again, compared to the chance you might take in having your dog have the biopsy done of his lymph node that your present vet recommended. I still try to not put my animals through too many tests or surgeries, if it is not going to do anything but give a definitive answer that the pet has a fatal condition or disease anyway.
Right now, I have a cat that has had two mammary tumors for going on four years (very small like a pea, but still worrisome), that my vet wanted to take out. I chose not to, as I feel that if they are benign, then she will be okay for as long as they do not grow, but if they are cancerous, it puts the cat through so much trauma, pain and stress and the cancer seems to grow faster, and the cat dies more quickly. I have a friend who has lost two cats to mammary cancer and she said she will never put another cat through what they went through ever again.
Each person must make their own personal decision, and if you feel comfortable just having your dog live the way he is right now, and you cannot afford much more, in tests, and treatments, etc. then I would not feel guilty about that choice. He may surprise you, and make a great comeback, and live many more years with you!
God bless you and good luck!