My cat is breathing heavy and deep...?

My husband and I just came back from a local vet. We have a male orange tabby who is approx. 8 months old. I had noticed for the past few days his breathing has been quite shallow and heavy. He doesnt seem to have been exhibiting any other signs that I am aware of. We were quite disatisfied with the vet, as she gave us quite a long list of "possible" causes, and we are basically in the same position as before we took him. She scared us quite a bit by telling us that 2 of the ppssible causes could be either Leukemia or FIP. She explained that both are contagious to other cats, and we do have other cats. This was very frightening to hear because there is no cure for either of these diseases. However, she did mention that he was not showing multiple signs of either of those diseases. Basically the only common symptom is his heavy breathing....which could be a number of things. We are praying that it may just be a respiratory infection or something very treatable because we have no money at all to help him. If you might have any adivse on things we may try or specific ways we might be able to say for sure if it is FIP or leukemia. Our main concern is getting our kitty to feel better, but we are definetely concerned for our other cats health and safety. Please any advise is appreciated. Also, like i said, the only symptoms that we are really aware of is that he is breathing heavy and shallow, and he seems to lay around alot, but he has been like that since we got him. I honestly do not know how is appetite is because we have other cats so its hard to tell.