Littles generally get a microchip implanted after adoption, like with babies sometimes, to prevent kidnapping. In the case of Littles, the Chip also deactivates any Portal they try to enter.
Maybe his chip is a special kind that can "pull" a little back to their mommy's dimension. And to escape that, he has to find a way to deactivate the chip. It could also have a timer, and he gets pulled back after a certain amount of time. Like, 5 hours or something.