Must One Stand When One’s Parent Enters the Room?
One is obligated to stand each and every time his father or mother enters the room, unless the parent explicitly granted “Mehila,” meaning, he/she waived the child’s obligation. If one is standing and knows that his parent will soon be entering the room, he should sit so that he could stand and fulfill the Misva of honoring parents. One must stand when a parent enters even if the parent suffers from a mental disorder and will not notice the display of honor. One who is sitting in a bus, train or airplane must stand when a parent boards.
Adopted from the shiurim of Ha Rav Eli J. Mansour