How to re-add a teen to Family Sharing without their consent?

Even if you can get him back I suspect you would have to set up restrictions again.

You cannot add him unwillingly. His account would have to accept an invitation to join and you would then have to set up restrictions as outlined in: Set up parental controls with Family Sharing on iPhone --> https://support.apple.com/guide/iphone/set-up-parental-controls-iph00ba7d632/ios

If he is not willing to let you do that then you may have to undertake more fundamental ways to make his device unusable until he will cooperate, such as was suggested by MrHoffman.

The other concern is, how was he able to use your device to do this, and what else might he have done on your device or learned from it?

Read this document if you think your Apple Account has been compromised. --> https://support.apple.com/HT204145

Use the information in this document to check your Apple ID device list to find where you're signed in --> https://support.apple.com/HT205064

Unless he has his own credit or debit card he may be finding a side effect acting on him in that a card may be required to use some third party apps and services. Once he left the family, your payment method will not longer be serving this role.

Consent is required. Non-consensual association would be immensely abused.

Or you can (temporarily?) remove the phone from the cellular plan, and wait.

Best assume all of your own passwords and passkeys are compromised, too.