You need to enable Screen Time on your Child's iPhone but you need to set Screen Time restriction on your iPhone and add him on your iPhone through Family Sharing
Use Screen Time with your family
With Family Sharing, you can share music, movies, apps, and more with family — and it now works with Screen Time. You can view reports and adjust settings for children in your family any time, right from your device.
If you're already in a family group, go to Settings > Screen Time, and tap your child's name. If you need to create an Apple ID for your child, go to Settings > [your name] > Family Sharing > Screen Time.
Or if you're new to Family Sharing, tap Set up Screen Time for Family and follow the instructions to add a child and set up your family. You can add family members any time from Family Sharing settings.
To use Screen Time with Family Sharing, you need to be the family organizer or parent/guardian in your family group, on iOS 12. Your child must be under age 18, in your family group with their own Apple ID, and on iOS 12.
Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support
https://support.apple.com/en-us/HT208982