Build up of fluid in my kittens lungs?

My husband and I recently took our kitten who is approx 8 months old to the vet because he was having breathing problems. The vet told us after examining him that he probable had either FIP or feline leukemia, neither with a good outcome. After taking an X-ray and withdrawing the fluid from his chest, the vet told us that by the looks of the fluid, he most likely has FIP. They also gave him a steriod shot of prednisone. This was yesterday at about 10:30am. Since then, he has become increasingly well and it looks as though his breathing is back to normal. He also had a fever yesterday, and was hot to the touch. Although we do not have a pet thermometer, he does not feel as though he has a fever anymore. My question is this: What else could have the same fluid characteristics as FIP??? The vet did not give us a definite answer on what he has, only a guess, and I would like to know if there are other things out there that may have the same fluid-like qualities. He also has a great appetite, has been going potty, and plays with the other cats. I am praying for a miracle here because this little guy is like a child to us. Any advise you might have on this would be so greatly appreciated. Also, the vet said that there is a chance that the fluid could come back, but also a chance that it might not. Now if he does in fact have FIP, would that be a guarantee for the fluid to come back again? I am learning more and more about this disease but I feel like there is much more to know. He seems to be doing so much better, but I just dont want to get my hopes up incase something terrible happens.