Tom is shocked and astonished when Humphrey informs Tom (the king) that he is the whipping-boy for the new king.
In Chapter 14, Humphrey informs Tom that because he is royalty,"None may visit the sacred person of the Prince of Wales with blows"; so when Tom makes a mistake, Humphrey takes the blows for it as his occupation demands. Incensed at this idea, Tom promises to put an end such a practice by terminating his days of study. However, Humphrey begs him not to do so because he and his orphaned siblings will starve if he loses his position as whipping boy because he is the sole provider for them.
Further, Tom realizes that Humphrey can be helpful in teaching him what he should know, so he encourages Humphrey to "remind" him of things he should already know. In this way he can learn what he needs to in order to continue in his charade as royalty.