Big Daddy (1999)

Sonny Koufax, a law-school grad who has been milking an accident settlement to cover his living expenses, while he continues to slack his way through life. But when his girlfriend threatens to dump him, he decides to show her he's serious about their relationship and pretends to adopt a little boy (in fact, his roommate's son from a one-night stand several years earlier, who shows up on their doorstep just after the roommate leaves town on a job). But after taking care of the tyke for a couple of days, Sonny finds that it's a little like feeding that stray dog that followed you home...