Hi there Steph,
Make sure you have age limit restrictions set for the App Store by going into Settings>General>Restrictions and enabling them and finding the App Store age limit restriction.
Sadly, the purchased apps cannot be removed from the account, but can be deleted off the device. I'm sure you know how to delete apps from the device.
To stop him from re-downloading the apps, you should change the password to that Apple ID, and create a new one. Change your password by following these steps:
- Go to My Apple ID (appleid.apple.com).
- Click “Manage your Apple ID” and sign in.
- If you have two-step verification turned on, you'll be asked to send a verification code to the trusted device associated with your Apple ID. If you're unable to receive messages at your trusted device, follow the guidelines for what to do if you can't sign in with two-step verification.
- Click "Password and Security".
- In the "Choose a new password" section, click Change Password.
- Enter your old password, then enter a new password and confirm the new password. Click Save when done.
Then create a new account for him to use by going to appleid.apple.com and creating a new Apple ID (You will have to use a different email address). Then just sign into iCloud and the App Store and anything that requires an Apple ID on the device. Note that you must be 13 years old or over to legally have an Apple ID under your name. If you are worried about him getting apps you don't entirely agree with him having on his device even with the restrictions set on his device, then it's probably best you don't tell him the password. If he wants an app, he can just bring the device to you and ask you if he can have the app, and you type the Apple ID password in for him if you allow him to purchase the app. Also, the old Apple ID you changed the password to hasn't been forgotten about, feel free to use it for yourself, or give it to another family member 🙂