You can restrict the usage of an application, service, or option through the built-in settings related to Restrictions. Apple created a page where you can get instructions on how to assign PIN codes to certain apps, thus restricting their usage. I recommend checking the page through the link posted below.
Note: Your son won't have the possibility to use the app when the PIN is activated, but he can see that the app has been protected with a PIN code.
Also, found a good article, recommended for reading: https://iphone.informer.com/iOS-Stories/setting-up-parental-control-on-ipad.html