pnemonia is a bacterial infection, the crackling is a sound vets will describe when they listen to the chest using a stethescope. When the cat get croupy, does he have a temperature (are you able to take rectally?), does he lose appetite, any vomiting (I had a cat with asthma, during an attack she would vomit -ipec worked wonders for her acute attacks, still treating the chronic) Its really important to know if his resp broblems are due to viral componant or a type of asthma.