Why not?
What happens when you try to invite him back?
do you get an error? If so, what does it say?
click here ➜ Invite people to join your Family Sharing group - Apple Support
Screen Time restrictions are not affected by being part of Family Sharing or not. You can remove Screen Time restrictions by accessing Screen Time directly on his device. If it was setup using a password, you will need to provide that password to continue and turn off restrictions or Screen Time entirely.
click here ➜ Use Screen Time to manage your child's iPhone or iPad - Apple Support