Must one inform or seek permission from one's mother before going for Umrah if the son fears she might disclose the news?

Legally, you are not required to inform your mother or seek her permission to go for Umrah, even if you do not fear any harm from informing her. Scholars have explicitly stated the permissibility of a son traveling for trade and Hajj without the parents' permission, as long as the journey is safe and there is no fear of perishing therein. If the mother becomes angry, then work to appease her through every legitimate means.