J.Z.
If I were you, I would tell him about it, but other than that, nothing. Preschool is about learning about the demands of school. You should not worry about preparing your child for preschool. It is their job to teach your son, in a setting where you are not present to learn to follow the rules of school settings. Relax and enjoy your son while he is home, and worry not. As school demands increase he will learn it is necessary to conform to certain rules in certain settings while still being himself. It will all work out, every 3 year old is active and they all have the same things to learn and the same experiences to experience, your child and you will be fine on this new adventure!
Good Luck and enjoy this new phase of life, it is so fun!
J.