Rivals (1972)

Peter is a wacky free spirit who runs a New York tour specifically for New Yorkers. Christine is an uptight art gallery owner, recently widowed, who is stepping back out into the dating world. He pesters her into going out with him and, since she cannot perceive the difference between obnoxious behavior and charm, she falls in love with him. There's a problem, though. Her 10-year-old son, Jamie is a loonball. He loves her way too much, and has an unhealthy and pathological attachment to his mother. After she remarries, his rage and misery overwhelm him and he plots to kill his stepfather.