Is it forbidden to eat from the lunch of a grandfather who does not wish his grandson to be present?

If your grandfather does not give you permission to eat, or gives permission out of shyness and not out of genuine willingness, then it appears that it is not permissible for you to eat from it, because permission requires the presumption that it is given willingly and not out of shyness. Likewise, if you doubt the satisfaction of a relative or friend with you eating from their property, then it is not lawful for you to eat from it.