Preventing modification of single select field

What's his permission level?  I believe if you make his permission level "Editor" instead of "Creator" or "Owner", he won't be able to make base schema changes (like adding field options). 

Is downgrading his permission level an option for you guys?

Thanks, but his permission level has always been "Editor." 😕😕

Unfortunately, this is not a built-in feature of Airtable.

Your only real option is to change the single-select field into a linked record field, and prevent him from creating new records in the linked record table. 

Alternatively, if  this is only a problem when he creates brand new records, you could create a form for him to fill out, because forms are the only part of Airtable that prevents people from adding new options into single-select fields.

You could also create a formula or an automation to alert him or you that an invalid entry was entered, but the best option is to just change the single select field to a linked record field. 

Depending on your relationship with your work partner, you could try creating an option called “Stop making new options and choose one that’s already here!” and then making that the default option for that field. That way, every new record has that option shown in it first and he has to click out of that in order to click into the one he wants. 

Is this the most professional and appropriate way to handle this? Absolutely not. Could it work? Absolutely.