You can generate a unique ID by using either an Autonumber field or a formula field with RECORD_ID().
As for guaranteeing that his ID shows up in other records you create, it depends on how he is creating records. Are you using a portal? Is he entering records via forms? Is he creating records directly in Airtable while logged into Airtable?
If he is creating records directly in Airtable, you can use a {Created By} field that will be associated with the user's Airtable account. You could then have a table that matches the Airtable user account to your ID and an automation to create any links. On the other hand, you would have to pay Airtable for every user.
If you are using a portal system, look into how the portal creates records.
If you are using Airtable forms, you could do some things with prefiiled fields and/or requiring the user to be logged into Airtable and using a {Created By} field.
