"Max already protecting mommy & his little sister!" Aguilera, 33, wrote beside the pic, which was taken by a paparazzo during a family outing in L.A. on Saturday, July 12.
The sweet snapshot features the mother-son duo walking and bonding in a parking lot. Little Max makes a mighty big gesture by holding his left hand over his mom's burgeoning bump, while holding her hand with his other.
Aguilera added text over the black and white snap, which read: "Big Brother Love." The proud mama concluded in her tweet, "He's the greatest. #maxthegreat."
Us Weekly broke the news of Aguilera's second pregnancy—and first child with fiance Matt Rutler—back in February. She shares custody of Max with ex-husband Jordan Bratman, to whom she was married from November 2005 to October 2010.
While preparing for baby No. 2, the Grammy winner has taken a hiatus from the judges' panel on NBC's smash reality show The Voice. "So blissful in taking this time for creating all things blossoming new on the horizon," Aguilera tweeted in April. "Album, baby & beautiful music to come."