How to set family sharing on two devices owned by a child

1. I am the family sharing organizer. Our son is the only member on that sharing. In his iCloud setting, it shows his devices include his iPhone, his MacAir and my MacBook. How do I remove my MacBook for his list of devices? When I click the "x" box for my MacBook, it reappeared when it is reconnected on the internet.
2. How do we set Screen Time on our son's MacAir?

Thanks for the reply. On the MacAir, I created a user account for my son. Is there a way to link that account to his Apple ID? I did not seem to find it.