Depending on the spread of the disease this may or may not be treatable.
You can read about mesenteric lymph node involvment in cancer in cats here
http://maxshouse.com/understanding_cancer.htm
and here
http://www.animalnetwork.com/petindustry/vpn/ooweb0209.asp
He should feel a lot better with the fluids drained as that puts pressure on lungs and heart and all the abdominal organs and is pretty uncomfortable.
If you are just working with your regular vet on this you may want to consult with a veterinary oncologist. Like the doctors who practice this the veterinary cancer specialists are usually really up to date on top treatments and can give a pretty solid estimate of prognosis as treating cancer is what they do. This site may help you find such a specialist to consult with
http://www.acvim.org/Specialist/Search.aspx
My experience in treating animals with cancer is that they do quite well in many cases and then if you get to a point they no longer enjoy life or are comfortable parting with them is kindest.
But with new treatments and protocols that point can often be delayed.
Hope this helps you!
__________________
Please press Accept if you find my answer helpful. Want more info? Just reply on your question!
Bonuses for good work are always appreciated!
Thanks for the opportunity to help you!

