He said the kidnappers demanded the equivalent of nearly $400,000, even providing an account number for the bank transfer, but eventually settled on 5 million naira (about $40,000).
"They said [it] should be paid immediately or the boy should be killed," said Ihemeje.
This is the third child abduction since the start of June and the recent kidnappings have caused outrage in Nigeria.
On Sunday night, unknown ransom seekers released Margaret Hill, the three-year-old British girl, unharmed after four days in captivity.
Armed men had snatched her on July 5 from the car in which she was being driven to school in Port Harcourt.
The girl's family and authorities in Rivers state, where Port Harcourt is located, said no ransom had been paid.
Umaru Yar Adua, the Nigerian president, said last week that no political or economic grievance could justify the kidnapping of a child.
The Niger Delta, which accounts for all of Nigeria's oil wealth, is in the midst of a political crisis since it has been neglected despite its economic importance.
Some armed groups have kidnapped oil workers and attacked oil facilities in an increasingly violent campaign to seize control of oil wealth. Some groups have carried out kidnappings just for ransom.
Nigeria's oil output is down by over 20 per cent because of these attacks.
About 200 expatriates have been seized in the Niger Delta since the start of 2006 and most of them have been released unharmed in exchange for money.
At least 11 foreign hostages are still being held by various armed groups in the delta.