Paul Fronczak, 49, was raised by Chester and Dora Fronczak after detectives found him abandoned in New Jersey in 1965.
But he questioned his identity as he felt he did not look like them, reports say.
Hundreds of police officers and FBI agents searched for the baby after his abduction from Michael Reese Hospital in April 1964 – when he was just one day old.
A woman dressed as a nurse reportedly told Dora Fronczak that the doctor wanted to examine her son. She handed him over and he was never returned.
Over a year later, a boy deemed to resemble the missing child was found abandoned outside a shop in Newark and given to the Fronczaks.