There are 2 Hercs at the Overlook. Indicated with "H" on the map.
Go get the one at the very top first. It's where you entered Thebes the very first time.
The other Herc is high up on the big shield. You can only reach it when you come back from the top and jump off the roof onto the shield.
Take all 5 Hercs back to the boy to finish the mission.