Apps are permanently tied to the account used to download them originally. Delete the apps and download them with your account. If he paid of them, look into using Family Sharing. How to share apps and purchases with your family - Apple Support
Deleting an app may or may not entail data loss depending upon where the data are stored. You don't really have much choice though.
As for the second issue:
Require a password for purchases in the App Store and other Apple services - Apple Support
1. Open the Settings app.
2. Tap your name.
3. Tap Media & Purchases.
4. Tap Password Settings, then choose the setting that you want (always require or require after 15 minutes).
About turning off password protection
- You can't completely turn off password protection for paid items on your iPhone, iPad, or computer. On Apple TV, you can turn off password protection for both free and paid items.
- If you enabled Touch ID or Face ID for App Store and iTunes Store purchases, you'll be asked to use Touch ID or Face ID for every download, even if the item is free.