Cyrus (2010)

Seven years after his divorce, John (John C. Reilly) has given up hope that he will find love again. Then he meets Molly (Marisa Tomei), and romance quickly blossoms. But she is unusually hesitant to bring him to her house and, when he secretly follows her home, he learns why: Cyrus (Jonah Hill), Molly's grown son, refuses to share her with anyone. John soon finds himself locked in a battle of wills with a possessive man-child.