The boy told jurors at his parents' sex abuse trial that he began having intercourse with his mother on his 8th birthday, about a year after his father began showing him pornography.
Prosecutors say the boy was unable to use eating utensils or tie his shoes when removed from his Plymouth, Luzerne County home in August 2010.
The boy's parents denied the allegations in testimony Thursday. His mother said her weight and a car accident had left her unable to have sex.
Closing arguments are expected Friday.
The Associated Press generally does not identify alleged victims of sexual abuse or their family members.