Confronting the situation head on is the answer. But the level of your friendship with the family might need to change before you do so. It sounds like you're not very close to the young boy or the parents, it's noble that you are concerned, but to avoid being called a busy body I would advise you to draw closer to the family.
If you invite them to do something like a bike ride and then go for a bite to eat you can expose the illness. You could also ask him to help you wash your car. I certainly think drawing closer to the family beofre you confront them is the best course of action. Show them you care, actions speak louder than words.
Kate xx