Patrick Schwarzenegger -- Hey, Rob Lowe ... Is This My Size? (PHOTO)

Patrick Schwarzenegger's got a new running mate -- when it comes to shopping -- and his name is Rob Lowe!

Rob and Patrick were having a blast shopping at an Yves Saint Laurent shop in Bev Hills over the weekend. Nothing shocking here ... just a 22-year-old bro and his 51-year-old homie checking out some overpriced clothing.