What is Jack's first reaction when he sees his father and what is occurring in the living room? Why can he not act on his impulse to run?

When Jack first sees his father surrounded by the family, he wants to run away. His legs will not move and his heart is pounding. He smiles what he calls a "grotesque smile" as he looks into the room, holding on to the doorknob, and finally says "Hi dad." When he approaches his father, he begins to feel dizzy and nauseated.