To Order/Sort the results that show up in the Reference field Picklist renderer, you should do 2 things:
1. Add a Model to the page to load in your Regions. With the Model’s properties, you can set the Order By clause directly, e.g. ordering them by “RegionID__c”.
2. Set your field’s “Option Source” to Model, and select your Regions model as the Option Model.
This community post describes this approach in more detail. In this post the goal is to be able to show more than 200 picklist values, which is another reason / use case for using Model as your field’s Option Source.
