My guess is: When you select to create a new world, there's a selection called 'More Options'. If you go to that, in World Options you should see a form called 'Seed for the World Generator'. It's been a long time since I've even played Minecraft, but this youtube video has some cool options: https/www.youtube.com/watch?v=U5Mh4S2NA_w
The seed is essentially a random string of numbers, which is then used to create a random iteration of the world that you want to create. That random world creation is then varied depending on other options you have selected in the World Options screen. If you look up seed generation styles either on Google or on YOutube I'm sure that you'll find one that you like and that your son will enjoy!