Daisy,
I'm so very sorry to hear of your troubles. The advice you have gotten so far is right on. First priority is, get his pain under control! If he doesn't hurt so bad, he will have more energy and incentive to fight this battle. You will both need all the fight you can muster.
And right now, this minute, start getting him to eat as much as he can put away, build up his weight, give him vitamins, get a dietary consult, whatever it takes to put the pounds and muscle on.
Best wishes, and keep us informed! We are all pulling for you guys!

Andrea