Drivers license silliness

So my and my husband did not grow up in TX (or, in fact in the USA, but we’ve lived in the US for 25 years and in TX for over 5yrs) Our kids followed the pattern for Gen Z and chose to learn to drive a bit late. Oldest was already over 18 and so took the route for adult drivers. Youngest will be 18 in a week, passed his road driving test yesterday afternoon and now we to get his permit changed to a proper driving license BEFORE he turns 18. So this week! During Xmas. First - WTF - what is this rule and why didn’t we find out about it sooner. More importantly - how do we even achieve this. If you know of DPS locations that release appointment on the day of - PLEASE share (preferably with 50 miles or so of San Antonio) Gonna try Boerne on Monday - but this situation is just silly