You could consider restricting either the ability to install or delete Apps - or perhaps both.
Here are the relevant ScreenTime restrictions:
Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Installing Apps
Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Deleting Apps
Either property can be set to Allow or Don’t Allow; the latter will inhibit this action.
It follows that you must set a ScreenTime Passcode that is known only to you - and not your son.
More information on available ScreenTime and Parental controls can be found here:
Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support
Use Screen Time on your iPhone, iPad or iPod touch – Apple Support
Set up Screen Time for a family member on iPad - Apple Support