In court Wednesday, the Franklinville man said he placed his son in a cradle, and didn't call for help until he discovered later the child was unresponsive.
The baby was resuscitated by emergency workers, but died a few hours after the May 2006 incident.
Duncan faces between 25 and 30 years in prison under a plea agreement with the Gloucester County Prosecutor's Office. Sentencing is scheduled for May 14.