Hi bh750,
Welcome to the Planet Coaster forums. You have come to the right place for questions about Planet Coaster.
WingardiumLevicoaster and breezerHOG have posted excellent suggestions. There is no 'simple mode' in Planet Coaster, but there are things that can make the game easier. Like the ones before me posted; you can start with the easy scenarios and just play with the game, or start with the sandbox which lets you play without money restrictions. From there, just play along while you and your son start to learn how the game works.
I personally compare Planet Coaster with an advanced box of LEGO. You can start with simple creations, keep your parks small and see how it goes. You will be able to pick up management in the game from there and slowly but surely improve the skills of you and your son to create bigger and better creations in the game.
It also depends on how quick your son will be able to pick up how the game works. If he likes simulation games in general and is easily able to learn how things work, then this might be a great game for both of you.
Another point that you need to keep in mind is that Planet Coaster requires a high end computer to run smoothly.
What I would like to recommend is that you watch some videos on YouTube to see how the game works, to get a better idea of how this game works.
One of the best things to start watching on YouTube are tutorials, such as the ones from
Silvarret.
Feel free to ask questions about Planet Coaster! Enjoy your time here on the forums and with the game!