Louis Tomlinson -- Baby Hand Off With Baby Mama ... Here, He's Yours Now (PHOTOS)

Fresh off his Mexican vacation, Louis Tomlinson got a fresh dose of reality when baby mama Briana Jungwirth handed over their 4-month-old son Freddie.

Looks like Louis is on daddy duty at least through the weekend ... the exchange with Briana went down Thursday afternoon outside a Starbucks in Thousand Oaks.

Louis' rented a home in nearby Calabasas for Briana and Freddie, so it's unclear why they'd make the exchange in a parking lot ... then again, when you're raising a little tyke, got to fit in those Starbucks runs somehow.