Same with the book sequel. I think Rosemary originally names him Andrew. The baby is then kidnapped by the devil cultists. They cause Rosemary to fall into a deep coma. Meanwhile, the son is raised by the Castevets as Adrian Steven Castevet. However, people call him Andy. When Rosemary emerges from her coma 30 some years later, in 1999, she learns that her son is now a great spiritual leader.
Stephen McHattie played him and the reason I remember that is because he kissed some girl and afterwards, the world's longest gob of movie slobber hung suspended between them for what seemed a grotesque eternity.
It scarred me, emotionally.