You should not worry about Chicken Pox. It helps to improve the immunity of his health and that is why it is considered as God’s blessing for a long life. And in such cases, missing one exam is not going to ruin his life. He will have to write many more exams in his life and a temporary set back is not going to cause any problem to him in the long run. Astrology cannot help you in giving any prediction on this count which lasts only for a few days. It is similar to missing a train due to a traffic block.