MaxMaks wrote:
Also, the main reason for him using mine is that he needs my thumb to purchase anything. Can that still be done using his own account?
I haven't examined everything in detail, but it sounds like that's possible.
Use Screen Time on your iPhone or iPad - Apple Support
Use parental controls on your child's iPhone and iPad - Apple Support
You can turn on Screen Time, and then use Parental Controls to keep him from making App Store purchases. You can also lock the Screen Time settings with a passcode (which you don't tell him), so that he can't simply turn off the restrictions.
You may want to consider using Family Sharing – with the controls set up so that he can't automatically buy apps with your purchase method, but instead has to go through Ask to Buy.
Family Sharing - Apple
Set up Family Sharing - Apple Support
Approve what kids buy with Ask to Buy - Apple Support