What is your model Apple TV? Identify your Apple TV model - Apple Support
Hidden movies should not be listed in your library. The movie catalog of everything that is on sale, will be available to browse. When found through other means, hidden movies would be shown as a not-purchased item. It may still show a trailer. It may still show alternative content providers (e.g. FX, fuboTV). Hidden movies shouldn’t be playable from there as iTunes Store purchase, though.
Additionally, you can set age restrictions for video content. That should prevent playback for content above the set threshold, by needing the Restrictions PIN number for that. Restrict access to content on Apple TV - Apple Support
Additionally, consider if his own account is something for him. Then multiple users on Apple TV allows each user to have their own TV app experience, and your library/queue list/recommendations wouldn’t be presented in his user space.
It also allows any optional purchases for him to not show on your account.