Justin Bulley died at his mother's home in February.
At the time, his father - who was trying to get custody of the boy - said Justin ingested fentanyl that was left around his mother's home.
Distraught father says baby died after ingesting fentanyl at mother's Lancaster home
The medical examiner has now confirmed the drug was the cause of his death and has ruled the manner of death as an accident.
No charges have been filed at this time.