Help Dirk!

I've had Dirk since he was a tiny little puppy. He has been through the best and worst with me. As most of you know, Dirk is having some serious medical issues. He was at the vet on Oct 12, and Oct 15th for some lethargic behavior, loss of appetite, etc. Neither visit were conclusive. They know he has a small amount of bloody fluid in his chest cavity but they don't know why. The vet wants to do more tests to figure out what is causing this. He is going to need a catscan and some additional blood work, on top of the 2 other xrays and ultrasound he has already had. Plus the other blood tests and fluid tests, etc. They think it's cancer but can't actually prove it, so I would like to get to the diagnosis so I can learn what I need to do to make dirk's life more comfortable. Thanks.