What does the whipping boy do in this story?

The term is medievil and progressed in to the 17th century in Europe (UK especially)

In those days males born of "proven stock", which basically means they were wealthy and from good families. These in the norm were princes at the least. If these young men did not follow their educational programme and started to fail they were to be punished. As it was a capital offense to hit a member of the royal line a young male was selected of the same age to have the punishment. They were normally struck across their hands or buttocks with a long narrow stick.

The term remains in place to describe someone who takes the punishment for others.