Details of the case have emerged in a ruling following a private family court hearing in Chatham, Kent.
Judge Richard Scarratt has not named the social worker but said she worked for Kent County Council.
The case came about after the father of the boy, who is now 15 months old, was convicted of raping a woman.
Social workers planned to place him for adoption but subsequently decided that he could live with a relative.
Judge Scarratt had approved the baby's move to a relative's home and said his parents would be able to stay in touch.
The judge said the couple had wanted to continue to care for their son and wanted him to be christened.
He said the father had taped a conversation they had with the social worker and complained about her conduct.
During the court hearing, the social worker had admitted "... that she had informed the parents that a christening which they wanted might reduce the pool of possible adopters."
Judge Scarratt told the hearing: "It is regrettable in my view that the social worker ... had indeed acted as the parents stated she had, discouraging the christening because it might hinder adoption."
He called for council bosses should hold an internal inquiry into the social worker's conduct.
The council has apologised to the couple but the Judge said the apology had been "too late and inadequate".
He has not identified the family involved.