The phrase "a plethora of cereal boxes staring back at him" means that there were
a large and overwhelming number of cereal boxes in front of him, likely lining a shelf or aisle in a grocery store. The use of "staring back" personifies the boxes, giving the impression that they are looking at him with an almost accusing or demanding gaze. This creates a sense of being overwhelmed by choice and perhaps a bit intimidated by the sheer quantity.
