Paramedics checked Alexander Draper, 14, before turning him over to the Maryland Department of Human Resources, police said. The two were found in Woodlawn area of Baltimore County, police said.
His mother, 49-year-old Jerri Gray, was arrested on a charge of violating a custody order, police said. South Carolina officials plan to pursue her extradition back to the state.
Authorities said the teen was at a "stage of critical health risk," but wouldn't provide details about his medical condition beyond his weight, said Greenville County Sheriff's Lt. Shea Smith.
Officials are making arrangements to bring the boy back to South Carolina, Smith said.
Investigators didn't know if Gray had an attorney and her relatives have not responded to messages left by The Associated Press.
The South Carolina Department of Social Services contacted deputies Tuesday after Gray failed to make a scheduled court appearance so officials could begin investigating claims of child neglect and endangerment.
The agency doesn't get involved in such cases based on a child's weight alone, but will take action in "cases where health care professionals believe a child is at risk of harm because a parent is neglecting to provide necessary medical care," DSS spokeswoman Marilyn Matheus said in a statement.
A judge ordered the teen taken into protective custody.
Gray and Draper had been last seen Monday at their home in Travelers Rest, a city about 10 miles north of Greenville.
Investigators said they are trying to determine why they were in Maryland.