I can only speak to prednisone use in humans - myself! The weight gain associated with it seems to be a result of it making some people VERY HUNGRY all the time. Fortunately, I have only been on it briefly and for emergencies, but it did affect my appetite very strongly! Maybe it's the opposite in cats - it just makes him not want to eat - have you noticed a change in the amount he eats? Interestingly enough, my brother, who has been on prednisone in similar circumstances to my own, had it cause a complete lack of appetite! If it is that different between two genetically-connected people, I wonder how common each reaction is in kitties!