Declan Hainey's remains were in a litter-filled cot at the home he shared with his mother, Kimberley, in Paisley, Renfrewshire, in 2010.
John Rodden told the second day of the inquiry into his death at Paisley Sheriff Court yesterday that he 'didn't know if it was a doll or Declan'.
Mr Rodden, who had helped decorate the child's nursery, said he had gone to the property with his wife, Elizabeth, and his sister-in-law Ann McNeilage. He left his wife standing at the door of the property and went inside with Mrs McNeilage.
Mr Rodden told the court he was too distressed to speak to a 999 operator after calling the service, but his wife had gone screaming into the street.
The postman had earlier revealed that he had renovated the flat so it was in an acceptable condition for Ms Hainey and her son to move in.
The court heard they had gone to the house after learning from a health visitor that Declan was about to be reported missing. Elizabeth Rodden told the hearing she never thought her grandson was in any danger from Ms Hainey, who was convicted of murdering him but released last year on appeal.
The grandmother also said she would never have lied to social workers to stop the child being taken away from the heroin addict.
She said: "If I thought Declan was in any danger I would have taken Declan off Kim. She loved him. He was hers and she just loved him. She was happy. They were just two wee peas in a pod."
The hearing continues.