What is the ruling on the the eldest son refusing to visit his stepmother, fearing his mother's displeasure, knowing that his father's marriage is public and the son does not object to it?

We advise the son to obey his father in visiting his wife, for there is no disobedience to his mother or harm to her in that. If he fears hurting his mother's feelings, he should keep the matter a secret from her. Visiting the father's wife is a form of showing kindness to the father, and honoring one's parents is one of the greatest causes for Allah's pleasure and entering Paradise, for the father is the middle gate of Paradise.