Thai medical officials said the boy's mother too had died earlier this month, but could not confirm whether she also had bird flu.
"He died of severe pneumonia after he was admitted on 15 January following a referral from hospitals in his province Sukhothai," a senior medical official said about the latest casualty.
Another six-year-old body who died on Sunday was the country's first victim of the bird flu virus.
The latest death came even as neighbouring Laos became the ninth Asian country to be hit by the deadly virus.
Officials said tests had confirmed an outbreak of bird flu among chickens in the country bordering Thailand and Vietnam.
"We sent samples to Hanoi for testing and we have received the result that it is H5, but we don’t know the strain," the official said.
The official, however, said there was no evidence yet that the bird flu had jumped to humans in Laos, though 3500 chickens had died mysteriously in its Vientiane province.
Meanwhile, as the virus crept to new frontiers, a worried Australia pressed into service sniffer dogs and x-rays to stop people from bringing in gourmet food and souvenirs with feathers from Hong Kong and Indonesia.
Officials on Tuesday said fishing boats were under increased surveillance and access to poultry farms was being restricted to keep out the disease.
"We are using a combination of detector dogs, X-ray machines and physical inspection of bags," spokesman for the Australian Quarantine and Inspection Service Carson Creagh said.