Little Boy and his Mom are characters who appear in 6 books: The Adventures of Captain Underpants, Captain Underpants and the Attack of the Talking Toilets, Captain Underpants and the Perilous Plot of Professor Poopypants, Captain Underpants and the Terrifying Re-Turn of Tippy Tinkletrousers, Captain Underpants and the Revolting Revenge of the Radioactive Robo-Boxers, and Captain Underpants and the Sensational Saga of Sir Stinks-A-Lot.
In book 4 you will see the little boy putting salt in Professor Pippy Pee-Pee Poopypants's coffee
The Little Boy usually notices things (such as seeing Captain Underpants and other unusual things) but his Mom doesn't believe him. In the future, Mom sees something, and boy says, "And I'm the one seeing a therapist?" In the far future, the boy is grown up and the Mom is still seeing things. The Little boy makes a cameo appearance in the film, Captain Underpants: The First Epic Movie were he is briefly seen in the crowd of students at Jerome Horwitz Elementary school.