About a Boy (2002)

Will Freeman is a good-looking, smooth-talking bachelor whose primary goal in life is avoiding any kind of responsibility. But when he invents an imaginary son in order to meet attractive single moms, Will gets a hilarious lesson about life from a bright, but hopelessly geeky 12-year-old named Marcus. Now, as Will struggles to teach Marcus the art of being cool, Marcus teaches Will that you're never too old to grow up.

Source: TMDb