The teenage boy's mother and two other siblings have also been admitted to the treatment centre to be monitored, health ministry spokesman Sorbor George said.
He told the BBC that eight healthcare workers "who are at high risk because they came in direct contact with the boy" were also under surveillance.
The BBC's Jonathan Paye-Layleh in Monrovia says nearly 160 people are now being monitored since the new cases were confirmed last week.
Radio and television stations have resumed broadcasting Ebola awareness messages, he says.
Civil society groups have also stepped up a campaign to get volunteers to be vaccinated against the disease in a joint US-Liberia Ebola trial, our reporter says.
On Monday, Liberia said the US had agreed to send two experts to the country to help investigate the sequence of the outbreaks.
ក្រុមបាល់ទាត់ជម្រើសជាតិកម្ពុជា បានយកឈ្នះក្រុមបាល់ទាត់លីកលំដាប់ទី៣ របស់ប្រទេសចិន Baotou Nanjiao FC ដោយលទ្ធផល ៣-១ ក្នុងការប្រកួតពានរង្វាន់មិត្តភាព 2015 Yunnan, Asian International Football Open Tournament កាលពីព្រឹកមិញនេះ។
