Once he is strong enough again, I urge you to get him vaccinated. Ask the vet the difference between innoculating him with a dead virus versus a live one. I have heard that the dead one is more effective, but that may just be heresay. Then keep him up to date on his boosters.
The shots will not stop him from getting the disease if he has been infected, but they will lessen the symptoms and give him an added boost to get over it. He must be a cat of courage, and I would still be vigilant with him because they do relapse. As I said the shots do not cure the cat, but they add a preventative measure that Crusty would not have otherwise.