I suggest making a table of users where the the field is the userid. The two fields on that table would be Userid and Region.
You already have a field probably called [Current user] on the detail table with the formula of User().
Make a relationship between the users table (the one) to the details table (the Many) and then lookup the Region.
Then you can have a from rule that says
When the region is blank and the Region lookup is not blank, change the region to the region lookup.
I suggest putting that region lookup on the form in a hidden section if you have any trouble with the rule not firing properly.
