A family spokesman said no ransom was paid to gain his release.
Abductions for ransom are commonplace in the Niger Delta but children were rarely targeted until the past month, which saw three child kidnappings.
Amadi's abduction on Thursday came just four days after a British girl of the same age was released by her kidnappers in the same area.
On Sunday night, unknown ransom seekers released three-year-old Margaret Hill unharmed after four days in captivity. Gunmen had abducted the toddler 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 money had been paid.
In June, the three-year-old son of a member of the Rivers state House of Assembly was also kidnapped. Nigerian newspapers reported that a ransom had been paid to obtain his release.