I am so sorry about your baby. However, if he still has an appetite and is lively then I would cherish that. Most cats lose appetite and a lot of weight if they are not well. Mine lost half his weight for no apparent reason although he had all sorts of blood tests and ultrasound. I had to force feed him for more than a month and still he didn't make it. And he was almost 12. So if he still eats on his own you are at least ok for now. You never really know with cats. I have another one who I rescued 8 years ago. He was tiny and still is much smaller than all of my other cats (6). He had asthma and suffered from crises, we almost lost him twice when he was little and had three or four more crises up to two years ago. He used to cough a lot all the years but there was nothing to be done other than cortisone when he was in a crisis. For the past two years and with no change in his food or anything else he has stopped coughing. He is all well now. So you never know. I cross my fingers for your little guy and pray for the best.
