The boy, whose name wasn't reported, was found to be "clearly underweight" by Mercer County Children and Youth Services caseworkers who visited his home in early June.
His 11- and 4-year-old sisters were found to be in good health and his 9-year-old brother was underweight but not as much as the younger boy, Mercer County detectives said.
Marcy C. Rader, 28, the boy's mother and his grandparents, Dennis C. Beighley, 58, and Deana C. Beighley, 47, were arrested Wednesday.
They were each charged with two counts of aggravated assault, aggravated assault of a victim under 13, unlawful restraint of a minor, false imprisonment, endangering the welfare of children and criminal conspiracy.
Since being hospitalized, the boy has gained 20 pounds, hospital officials said.
"The most important medicine used to treat him at the hospital was food. He was within a month of having a major cardiac event that he probably would not have recovered from," Dr. Jennifer Wolford of UPMC Children's Hospital Child Advocacy Center, told the Sharon (Pa.) Herald.
"It is impossible to me that this severe neglect and active abuse was not visible. He was being starved in his own home around others of normal weight," she added.
The boy had been removed from public school and was being homeschooled. He was only allowed outdoors on the back porch of the house, where he would hunt and eat bugs.