I would definitely tell him in advance, to give him a chance to process the fact in time, and to say goodbye to the cat.
Morah made a good point about leaving the sickness out of the explanation, that may be one strategy. However, my feeling is that telling that the cat just died, without any clear reason, may be equally frightening for the kid if he has a tendency to get frightened by such things.
So I would rather explain that the cat died because it was very severely ill, and the doctors couldn't save its life; that there are different kinds of sicknesses, some just make your nose run for a few days, others may make you sick for longer times, and need doctors' assistance and medicines to recover. And sometimes even doctors can't help, and one dies, which means that it goes away, and we won't ever meet again.
I think a lot more depends on your mood and internal feelings than the exact words you use. If you feel uncomfortable talking about death, your son will sense it and it will make him agitated or fearful too. In Western culture, there is a lot of fear about death, and this is the way we pass it on to our children. We don't talk about it, and we hide our own dying relatives and pets out of sight, into darkened hospital rooms or the vet's lab. But our feelings we can't eliminate, only repress. And the children sense them anyway.
So if he asks, I would not hide the fact that all of us, people, animals, plants, will die one day. But for him (and us), it's going to happen many, many years after yet. However, I agree with Morah that there is no need to try to explain him too much. Give him the basic facts and then let him ask questions. If he asks a tricky one, like "where does the cat go when it dies?", you may ask back "what do you think?". Then continue the answer depending on the level and direction of his ideas.