Hey Jackie_N10,
If you have family sharing set up with you as the organizer and your son as a member, as outlined in this article: Set up Family Sharing - Apple Support, you can sign into your sons Apple ID on his iPhone without losing any data. This article explains how to sign in: Sign in with your Apple ID - Apple Support
We'd also suggest reviewing the information here for steps to use parental controls: Set up parental controls with Family Sharing on iPhone - Apple Support
"Set up Screen Time for a child later
Screen Time allows you to manage settings for downtime, app use, contacts, content ratings, and more. To use Screen Time, your child must be using an eligible device. See the Screen Time section in the Apple Support article System Requirements for iCloud.
- Go to Settings
> [your name] > Family Sharing > Screen Time. - Tap the child you want to set up Screen Time for.
- Tap Screen Time, then follow the onscreen instructions.
- For information about Screen Time settings, see Set up Screen Time for a family member on iPhone.
- To learn more about the different content & privacy restrictions, see the Apple Support article Use parental controls on your child’s iPhone, iPad, and iPod touch.
If your child requests more screen time, you can approve or decline the request in Settings > Screen Time or in Messages 
"
Thanks!
