Lebanese security officials said on Sunday that Rami Ali Hussein Shibly and his brother, Khodr, nine, were picking olives in their family's grove in Halta when the cluster bomb exploded.
The security officials spoke on condition of anonymity because they are not authorised to speak to the media.
The United Nations and human rights groups have accused Israel of firing as many as four million cluster bombs into Lebanon during its 34-day war with Hezbollah that ended in a UN-brokered ceasefire on August 14.
UN demining experts say up to one million of the bomblets failed to explode immediately and continue to threaten civilians, especially children who can mistake the ordnance for batteries or other small objects.
Sunday's death brings to at least 21 the number of people who have died in cluster bomb explosions in Lebanon since the war ended.
More than 100 people have been wounded, according to the UN Mine Action Centre.
The London-based advocacy group Landmine Action warned in a report issued this month that civilian casualties would continue to increase as people return to their villages in southern Lebanon and sift through the "cluster munition-infested rubble" of their homes.